yeah thats pretty much what I am doing with the TK860's, install accessory cable.. jump audio out from receiver to audio in on transceiver, then jump the TOS/COS output from the receiver to the PTT input on the transmitter.. Program the radios appropriately, hookup a tuned duplexer, antenna and enjoy.
Fundamentally its not more difficult than hooking up and programming these radios in the first place, getting it all into a nice portable size is where you need to be creative.
My repeater will typically be wired in at my house, but I will often hook it up either in my jeep or standalone at a base-camp.. Since both the jeep or camping will require some weather hardening I am using the ammo can.. i dont think it'd be too big... it'd fit on the floorboard behind the seat of most coupe's, sedans and up.
you dont need to put a AC power supply in it either if its just going into your car... leave the power supply external to save space and leave it at home.. you wont need it in your car.
The radios can be had for $60 or less
Duplexer Tuned off ebay is $80 shipped
Programing cable is $10 shipped off ebay
Accessory Cables are $15 each
Ammo can is $10 at GanderMoutain
about $250 total plus a few lil pieces
-R