GMRS install: hidden / hardwired Midland MXT275 GMRS walkie talkie + external antenna

[Ervan]

Sharing a recent install I just completed in my 2022 R1T Quad.

I decided a handheld walkie just wasn't enough -- so I put in a hidden, hardwired Midland MXT275 under the front passenger footwell!

I wanted a clean, no-show install for when not off-roading and also didn't want to take up any cargo space (like under rear seats), so mounted it under the dash/front passenger footwell, which also meant running less cable or a separate speaker. I used a more shielded antenna cable than most seem to be and I don't get any interference that I can tell.

Radio body is under the footwell and the Midland MXT275 has controller in the mic.

Mounted the radio behind the passenger footwell up next to the fuses, very close to where you can run wires through the firewall grommet. I'm a big dude (6'4", 280lbs) so working in this space was challenging, but with a small right angle drill you can drill into the plastic frame and screw the radio mount there which then the radio snaps in and out of. There's just enough space out of the way of things for all bits and cables to fit with reasonable amount of room.

I faced the speaker out, plugs go to right so more space under fuse panel.

I removed the full width dash panel (remarkably easy, a few screws, little popping action, boom!). Then I panel mounted a RJ45 Cat6 cable outlet under the dash behind the 12v outlet. Inside the dash it just fits between the airvents under the dash. I used a stepped drill bit to make a round hole then cut/filed square to pop it in.

Then, I installed the right side M&R ditch light mount which mounts to the body (very, very stable and well made) and ran an NMO cable through the firewall up to the mount and antenna. Midland antenna fits nicely here.

The NMO antenna cable and 12v power for the radio are through the firewall grommet (which is just left of the air vent filter intake, which you need to pull out to get to the grommet). Running the cables is a nightmare if you have the plugs on or a large cable, so either be good at cutting then reinstalling the plugs or use a lot of lube!). I used a stiff wire feed / tape thing from Amzn (see parts list) which helped so much -- as did a 9 year old kid to help fish things out as you push them through the grommet from the frunk side):

You can feed the cables sort of up through the cover panels along the body edge and is a pretty clean look afterwards.

Parts list (no affiliate links):
MXT275 MicroMobile® GMRS 15-Watt Two-Way Radio - MXT275 ($170)
Midland - MXTA26 MicroMobile 1.5 inch Base Diameter 6DB Gain Whip Antenna ($60)

M&R Body-Mount Ditch Light Brackets ($110 for L & R)
C101NMO Antenna Mount Cable Assembly, NMO, 6ft - Shielded ($50) - extra shielding and fits the ditch bracket perfectly
Qaoquda Cat5e RJ45 Square Flush Mount Cable, RJ45 ... Shielded Panel Mount ... 1M/3Ft ($13)

NEIKO 11402A Stubby Drill Bit Set for Metal, 5 Piece ($16) - for drilling in the footwell plastic frame in tight spaces
Fish Tape Wire Puller ($14) - to get through the firewall grommet


Many thanks to so many brave souls on this forum who posted pics of their Rivians in various states of disassembly, especially @HighVoltOverland (here) & @zonehawk (here) for their posts. Couldn't have done it without y'all.

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[robd]

Nicely done!

 

[SwampNut]

Running the cables is a nightmare if you have the plugs on or a large cable, so either be good at cutting then reinstalling the plugs or use a lot of lube!).

Are you aware of, and did you use, the cables that have a short ultra thing pigtail with a micro-connector attached, that goes to a full size adapter? I've always used those, love them. And can you describe the area inside the cabin where you find the firewall grommet? How to find it from inside?

Also if you don't fix "GMRS" in the title I'm gonna come give you a wedgie.

 

[Ervan]

Are you aware of, and did you use, the cables that have a short ultra thing pigtail with a micro-connector attached, that goes to a full size adapter? I've always used those, love them. And can you describe the area inside the cabin where you find the firewall grommet? How to find it from inside?

Also if you don't fix "GMRS" in the title I'm gonna come give you a wedgie.

lol thanks - typing too fast. Wedgie averted.

I saw those cables but wanted one that was very shielded given some people’s experience with interference and couldn’t find one like that so had one that goes from medium size adapted to large. Running those would have been way easier

For the life of me I couldn’t access the interior firewall grommet and only could get to it from the outside. It’s wayyyyy up inside the passenger footwell on the left side of the fuse panel.

 

[Ervan]

How to find it from inside?

On this, I found it way easier to use a rigid wire puller from frunk side - push through then can fish or pull wires either way. The one linked in parts list was reasonable and cheap.

 

[Rivian Owner]

A wire coat hanger works well as a rigid fish device. Just bend it straight

 

[Ervan]

A wire coat hanger works well as a rigid fish device. Just bend it straight

Also this - I found to pass cables with larger plugs the wire puller helped (it had like a little flex funnel that could envelop the plug) but hanger was much easier to get through the grommet.

 

[Rivian Owner]

Also this - I found to pass cables with larger plugs the wire puller helped (it had like a little flex funnel that could envelop the plug) but hanger was much easier to get through the grommet.

I find just wrapping electrical tape around the wire/connector/coat hanger will act like a "funnel" to help ease the connector through the grommet. Call me cheap, but I've got a garage full of tools and I don't need more single use tools. If you need more flex between the hanger wire and the connector, you can use a piece of mason's twine or equivalent to help you. A lot of times, I'll pull an extra piece of mason's twine with the wire so if I have a need for another wire to be pulled I've already got a pull string.

You mentioned the antenna cable you used, and I'm puzzled? I normally see coax/antenna cables designated such as RG 58, RG8, LM 400 whereas NM is normally designated as the connector between the cable and the antenna (normally for higher frequency RF signals). Since the run is so short it probably doesn't matter, but one thing you can do to avoid pulling the connector through the grommet is to install it after you've pulled the cable through the firewall; so knowing the cable designation and the connector type you can order the individual parts and build your own cable and not have a lot of extra cable to coil up. You can also use an SMA cable connector which is much smaller, and get adapters to go from SMA to NM style connectors. BTW, coiling extra cable up can have an impact on how much energy is transferred between the radio and the antenna.

 

[Ervan]

wrapping electrical tape around the wire/connector/coat hanger

Yeah, tried this a few times and the grommet was so tight it kept pulling the tape off / disconnecting from cable. My first time working with firewall grommets so not sure if I wasn't puncturing it adequately or something.

You mentioned the antenna cable you used, and I'm puzzled?

I looked at buying cable and connectors, but didn't want a bunch of extras since I don't wire this type of stuff usually and couldn't find a single short cable + single connectors to buy that didn't also require me buying other tools or having a bunch of stuff leftover.

I managed to find a cable that was exact type (RG316 Teflon), length (6ft, no extra spooling), and connectors (3/8" NMO, PL-259) that I wanted so took the pain on wire pulling. I tend to pull enough cable in other DIY home stuff that made sense just to get one.

 

[Oldsmobile_Mike]

Looks professional. Nice work!

 

[MukilteoManiac]

Great work. I am going to do this same radio install and antenna mount in our 2026 R1S, but with the little stubby Midland Ghost antenna instead. Thanks for documenting your installation!

 

[Ervan]

Great work. I am going to do this same radio install and antenna mount in our 2026 R1S, but with the little stubby Midland Ghost antenna instead. Thanks for documenting your installation!

Amazing! Can’t wait to see it. I also love those stubby midlands s.

Pics when you are done pls!

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