The unrealized problem in wiring for a trunk battery is with the car running, you have two positive sources running the car. One from the battery, and one from the alternator. If you wire it such that it turns off the battery, the alternator continues to run and power the system*. So, you either have to cut the field power to the alternator to turn it off (which requires either a 4 pole cutoff switch or a relay/solenoid) or wire the alternator output to the + battery making the car run on a single positive source.

I might add also that if that alternator output has any other connection to anyplace else other than the + battery you run into the dual + supply again.

Many people wire it wrong as in the first instance, but turning off the switch "works" at idle as the power draw is greater than can be supplied by the slow speed alternator. But note the asterisk below. More than one person has had tech inspect the cutoff at high idle and the car shuts off due to fried "stuff". ending the racing day right there in tech. Seen it, not on my stuff.


*Running the car with the battery cut out can allow the alternator to put out way too much power. I've measured 20 VDC at idle, but not for long, the MSD will die.