I always figured the engine block to chassis ground had to be the same as the Alt to batt., since the Alt case to alt bracket serves as the alt ground...or am I totally wrong here?
Technically that's 2 of the 3 wires combined. Factory wiring goes from battery negative terminal to the engine block (1 wire) and from battery negative to the body (another wire). You are correct that the first wire also carries return current from the alternator, but starter current is the determining factor in the stock wire gage. Getting the alt/bat current to the chassis is the function of a separate wire and it's often pretty small - I think my 85 Olds had a 10 gage there. So in terms of priority, I'd do the skinny ones first.
totally agree with maylar here. if you think about it, you use the pos from battery to amp and then ground the amp to body, so this is where the circuit (body to battery) is completed. i actually experienced this with helping the dimming issues on my son's car before doing the other 2 a few days later.
