After decades of working on car wiring systems, I've come to the conclusion that the majority of the problems are 1) poor connections, and 2) poor grounds. For your case, you say you have 12v at the ignition feed with the ignition off and 7v with the ignition on. That tells me you have a poor connection prior to the ignition switch. With the ignition on, just start following the wiring connection by connection back to the battery. Somewhere you will get your 12v back and there is your problem. The trick is to systematically start and work through the system in sequence. "Shotgunning" is rarely successful.