08-22-2021, 11:14 PM
I was hoping 69 Mach1 might chime in, as I believe he had worked through this on his Mach1. My Mach1 didn't have a factory tach. I mounted one on top of the steering column. So I can't be of any help. But it would seem your thinking is correct. As 12v + is 12V+ no matter where it comes from.
JTS
JTS