Hi GUNewbie, as a number of members have posted, don’t use the chassis as the earth return, it’s simple playing Russian Roulette you negative supply.
While “most” chassis earth returns will work, there is no way to test in advance of an installation if the chassis is a suitable HIGH CURRENT circuit and just posting that vehicles come from the factory that way doesn’t cut it.
Vehicle manufacturers know exactly where they can get an earth return and even then it’s only for very low current devices like light globes.
There are very, very few vehicles that come from the factory that have high current requirements anywhere other than in the engine bay.
Furthermore, the newer the vehicle the greater the chances are for a bad earth return through the chassis.
As to the cable, if you want to provide both the current and voltage at a decent working level for you compressor, you need to run much thicker cable than the 6mm.
6mm auto cable, which is actually 4.5mm2, will cause at least 1.5v voltage drop while trying to run a 25 - 40 amp compressor.
You need to run at least 10mm2 twin or better still 6B&S twin ( 13.5mm2 x 2 ) and you can then use it in a dual battery system if you ever go down that road.
Also, you don’t need to protect the compressor as it should already be fitted with it’s own suitably sized fuse, so if you go with thicker cable, best protection would be a 40 amp auto resetting circuit breaker on 10mm2 cable and a 50 amp CB on 6B&S cable.