The key is the Model T coil. A Model T coil has it's own "points" called a vibrator. Complete the circuit and the normally closed points open, the field collapses etc. and you have high voltage. Google "Model T coil" to see how one works.
Grounds, yes, very important... But as my master electrician Dad taught me, low voltage DC wiring needs to be treated a little differently then higher AC voltage. The idea of having a ground bar on both ends of the car is a very good one, but you need to take it one step further. Clamp screws work well with 110 AC, but for 6 (or 12) volt DC, you want to make all your connections with crimp on terminals or better yet, solder everything.