Short version: Yaesu FT-8800R (dual band 2m/70cm, dual receive, with remote mount head), Larsen NMO 2/70B antenna on roof rack mount, with Kenwood KES-5 speakers inside & outside on roof rack. I also have a Yaesu VX-170 and a Yaesu VX-7R handheld (both with upgraded Diamond SRH77CA antennas).
Long version: I currently use a Yaesu FT-8800R in my service vehicle, with the body mounted with a large slab of heavy duty velcro under the 2nd row seat, and the remote mount head installed up front in an open dash spot. The mic connects to the remote mount head, which is convenient, and the remote mount head receives its power from the radio body via the remote mount cable. The main radio body receives power via either 2-gauge or 4-gauge cable (I can't remember which) hardwired directly from the battery (via a 100A fuse at the battery) to a fused distribution block under the 2nd row seat, which is then run with 6-gauge cable to a Red-Dee-2 PS-75/45-4 (1 75A PowerPole input, 4 45A/30A/15A PowerPole outputs), and one of the Red-Dee-2 outputs is then run with dual-fused 10 gauge cable to the radio body under the 2nd row seat.
Sidenote: the other 3 Red-Dee-2 Powerpole outputs run a Kenwood 110W business band 150 MHz radio (also with remote mount head, and NMO single band roof rack antenna cut for 151.625 MHz Weatherman frequency) for offroad races in Baja, a 400W continuous inverter, and there is one spare output for future expansion, or powering an additional radio if need be. Each of the 4 PowerPole outputs have 10 gauge cable runs with the appropriate fuse in both the positive & negative runs of the cable. I like this setup, because it uses the appropriate size cable & fuses at each step of the way, and is flexible, since e.g I can unplug all aux equipment by simply unplugging the 75A power cable connector from the Red-dee-2, or just one piece of equipment by unplugging its corresponding output cable run from the Red-Dee-2. And since it's properly fused at each step, even if one fuse blows, the other equipment should still function properly. Also, if I plan extended use for rallies, and don't want to run my engine or risk draining the batteries, I can throw an extra charged AGM battery in the back, and I have a 10 ft 8 gauge power cable with battery ring terminals on one end, and a 75A Powerpole connector on the other end, which I can plug into the Red-Dee-2. So I can run all my aux equipment off a second battery or power source, instead of vehicle power.
I have Kenwood KES-5 weather resistant 4 ohm speakers, both inside the vehicle and outside on the roof rack. At the moment, I have to manually plug in either the inside or outside speaker into the back of the main body under the 2nd row seat (both speaker plugs are routed there). However, I am considering rigging up a switch harness that would allow me to switch between inside or outside speakers, or both at the same time. But that is an optimization, and manually plugging one speaker or the other into the back of the radio works fine for now.
Since I have a roof rack, with a huge spare tire & other obstructions on the roof rack, I currently use a Diamond K series roof rack antenna mount, with a Larsen NMO 2/70B dual band antenna, that is half wave on 2 meter. Despite being roof rack mounted, as opposed to the preferred install of being permanently mounted in the center of the roof with the whole roof as a ground plane, so far this setup has worked fine for me.
What I like about the Yaesu FT-8800R is that it's dual band receive, which means that one can listen to e.g. net frequency on one side, and stage frequency on the other (when working a rally), or e.g. net frequency on one side & rally car frequency on another (when servicing at a rally). However, as pointed out previously, cramming 2 radios worth of buttons onto a roughly single DIN sized remote head makes for a slightly cluttered interface. And there is also the annoying Internet WIRES function which gets activated if you press & hold the left side lower control knob which sticks out (a lot of people accidentally activate this when they first get the radio). Luckily it's easy to deactivate by just pressing & holding again, but it would be nice if it were possible to simply disable the WIRES function via a menu. Anyway, it sounds like I am complaining a lot about the Yaesu FT-8800, but really it's a great radio, with great performance, and lots features, that has a user interface that is fine when the vehicle is at a standstill, but for certain features can be a little fiddly in a vehicle at speed on a bumpy surface.
I also have a Yaesu VX-7R handheld (quad band, with upgraded Diamond SRH77CA antenna), which I could use to cross-band repeat with my Yaesu FT-8800R. However, I have only made use of that feature once. Most of the time it's possible to park the vehicle close enough to the stage start / finish / blockage area where it's possible to hear the radio, and easily step back over to it if necessary. I also have a Yaesu VX-170 single band 2m handheld (also, with upgraded Diamond SRH77CA antenna, predecessor to the current Yaesu FT-270R), which I actually prefer using to the Yaesu VX-7R since it's single band and therefore has a less cluttered interface that is easier to use, though it obviously doesn't have some of the fancy functionality that the Yaesu VX-7R does. Having a handheld can be useful in a number of scenarios e.g. I throw one in my bag when competing as a co-driver so I can listen in if a rally is delayed, or I can walk around service & still hear what's going on, or even use when working a rally away from my vehicle (though one's transmit range won't be as good as a mobile, unless using cross-band repeat, due to the lower TX power of a handheld, so not quite as useful as a full mobile in that capacity).