I need 12v

Ah..........yes. If you put it ONLY in the EFI wire that would limit the current you need. I would measure it. Doesn't that system have a "full time" 12V and then an ignition trigger line? That trigger likely does not draw much. There are some diodes NAPA may have, but finding something they have and you want may be tough. I'd just get on line, even Egag, about a 5-10 amp rectifier at 25V PIV or more is enough.

If that is the case, you would want to hook the diode to coil+ (as if it was ignition switch and the other end ONLY to the EFI power trigger wire

Then leave the wire on the EFI side of the diode connected to IGN1