I'm already a little bored with the HT performance and taking the plunge on a mobile install.  Here is what I'm thinking... please comment and help fill in the blanks!

Truck is a 2017 Silverado 1500 LTZ with sunroof.  There is little to no room under the seat due to the power seats and ventilation fans.

Radio - Kenwood TM-V71a based primarily on recommendations here.

The plan is to mount the radio under the back seat and the control head on a 24" RAM no drill gooseneck which will put the controls and display right around my center console.  Microphone will run into the center console area.

Antenna - Sti-Co Flexi-whip, drilled NMO mount.  I'm not sure what to buy for the NMO mount... Recommendations on brand & size please!  I only have 3 inches of clearance in the parking garage at work and want to run the radio on my commute.  I'll likely get a bigger antenna for road trips.

Remote Speaker  I have no idea.  Please recommend a speaker and where to mount it.  I've seen one mounted under the headrest and that looked cool.  I also figured either driver or passenger side under the dash or under the seat if there is room.

I've got 2 options for installers.  First is a local car stereo place that does a lot of high end customization.  They've done work for me before and I've been very happy.  They drill roofs for lights and antennas all the time and would do a killer job of installing the radio, etc.  Second is a proper 2 way radio shop that does a lot of commercial and school bus work.  They have way more experience with NMO mounts but way less with working on higher end trims and the clean install I'm looking for.

Overall thoughts?  Am I missing anything big?

The retailer I'm buying the radio from has an option for a MARS mod.  Is this something I should do?

I have the same model truck.

Let me know how it works.

I wouldn't add MARS unless you are active on MARS in VHF/UHF.

Radio + NMO sounds like a good plan.

Personally I'd use the radio shop and just point out where you want the coax to come out. The
rest (gooseneck and radio placement) you can DIY. Or if you want to split the difference let
them do the main unit and main unit 12V power install and then you can handle the head unit
and mic. That will keep them away from the trim, and honestly with the route you're going they
probably wouldn't touch any trim outside of getting access under the headliner and the cab wall.

General rule of thumb is keep away from the dash and everyone is happy.

I have a selection of NMOs depending on what I'm up to, all Larsen.

Skip the MARS mod.

Only 3 inches of clearance is the weak link. No way I know of to mount an an antenna on the roof so it will clear.So it will have to go on the bed. Ground plane will suck wherever it is mounted. The better option is an L-bracket between the fender and the hood. A no ground plane on a nmo on the L bracket will work. Just won't look that good.

Plenty for a Nitol whip such as the Flexi-whip. You can tie it into a knot. Literally what the antenna is made for.

OP, check out the Panavise InDash mount. It's a no drill mount which is pre-drilled to accept RAM pedistals. I have two in my Sierra.

Exactly my reasoning.  The clearance issue is really only the entrance "bar" and a few pipes; I won't be dragging the antenna all the time.

Now that is cool.  I might have to give them a shot.  I'll be using a RAM X-grip to hold the control head so that I can take it off in high theft areas; hence the need for a RAM ball.

Dude... been looking for something just like this to mount a new mobile unit

I used a sti-co flexible nmo and it didn't work worth a damn. Not a fan. Went back to a 1/4 wave antennex

I used a Sti-Co antenna for several years, I had a little CUV that had like 3-4" of clearance to my garage door. Antenna held up fine and worked the same as any other 1/4 wave.

When you buy them they come long enough for like 130MHz or something, you have to cut it to the frequency you want. I think they had a cutting chart or you could use an antenna analyzer to get it exactly on frequency. They give you a little rubber end cap, the cap changes the frequency slightly so if you use an analyzer test it with the cap on. A little drop of silicone glue or hot melt will stick the cap on when you're done.

As far as the NMO mount, really any reasonably made one will work fine. There are specialty ones made for GHz+ frequencies, and even some with a larger center contact that they claim works better on low band, but I wouldn't worry about either of those types.

The only oddball thing I might consider would be a "fully enclosed" mount. Those are a little different with a connector on the mount and then a mating connector on the coax. The only thing the extra cost and complexity gets you is a little reduction in signal levels inside the vehicle, but it might be helpful in reducing interference to and from other in-vehicle electronics.

Have you considered going with a DMR radio? Perhaps a CS800D? I have no idea what the VHF scene is like in your neck of the woods, perhaps analog is all you need, but around here it's dead.

Radio has already been ordered.  I don't really know a lot about DMR, but the VHF / UHF repeater network in the greater Houston area is pretty well established.

I'm curious - why do folks not route the speaker through their stereo?

Is there some technical or practical reason it's not done?

With most stereos there's no mixer on the inputs, so you're either on the 2-way
or stereo (e.g music) but not both. I'll listen to music and turn it down if there's a call that matters.

Mine is. I have an F150 and usually use the external speaker but if I'm having a good conversation I switch over to aux input.

Same here in the car.

I have been debating on upgrading the truck stereo to handle a dual in. Or doing a dedicated set of 6x9's for the yaesu. (with their own small amp and everything.)

In some vehicles, the speakers are notorious for back feeding stray RF (even with VHF/UHF).  Some vehicles on the road today simply do not have aux inputs. Sometimes you also run into issues where it's not as simple as plugging a 1/8" TRS jack into the back of the transceiver and then into the radio due to the level of the amplified output from the transceiver and the input of the stereo.

I do because the radio is under the rear seat and I can't hear the speaker under there. The head is up front and has a headphone out that I plug into the aux.

it works well

Not interested in re-wiring, so unless the VHF/UHF radio bluetooths the signal to the vehicle radio like an iPhone, not gonna happen.

Often I want both on, vehicle radio on, and VHF/UHF without cutouts everytime someone keys up.