Painless wire harness install (MORE PROGRESS)

The problem is that if you just kill the battery, the car will continue to run "on the alternator." I'm always bitchin' about guys who "test" an alternator by yanking off a battery cable. The assumption (incorrect) is that if the engine dies, the charging system is not working, and if it keeps running, it is OK.

So if you were to wreck, the engine still running, and are incapacitated, and some safety guy gets brave enough to yank the battery disconnect, the damn thing will just keep running

That's why you need the other switch poles (small terminals)