If you pick a good chassis or frame ground, there's no electrical difference between that and going back to the battery. In fact, it will be better than running a wire back, the chassis/frame is a fawking HUGE conductor, and will present less resistance. Also remember, you will be running this off the alternator, not the battery, most of the time. But the wires between the alternator, battery and chassis are rated for the full 140/160A output of your alternator. The chassis itself is a big and efficient enough conductor that it's current-carrying ability is virtually unlimited.
+1 on the solenoid and breaker. You can use the left-side cigarette lighter outlet to activate the solenoid -- it goes on-and-off with the key -- possibly with a switch in series, mounted on the dash, so you can keep the inverter off when not needed, even when the key is on. Keep the breaker and solenoid as close to the battery as possible, this way you're not sending power down the hot wire when you don't want to, and if you do get a short the breaker will be as close to the power source as possible. You'll also need crimp-on or screw-on lugs for the end connections, as wire that size is virtually impossible to solder.
Personally, I would go with 2 gauge, but I like to over-engineer things. According to the wire capacity charts I have, 4 gauge is good for 3% drop up to 15', 2 gauge up to 25', both at 80A. IIRC, you need to base this on the total round-trip distance, not just the one-way distance from the battery to the load; but I'm not 100% certain if this applies when using a chassis/frame ground.
I'm a JKO Reject
Last edited by BumpInTheRoad; 10-29-2010 at 10:56 PM.