Thanks Jonathan! That answers my question, and I definitely have some issues with the factory wiring to track down then. Strange how my fathers' car also has very low voltage there as well.
In looking at the factory manual, I see there is also a diode located somewhere in the cabin side of the circuit, which has me wondering if it has seen better days. Should be fun trying to find the darn thing though. Worse case, I'll run a new switched source through the adapter, but I would rather get it working correctly.
1985 Chevrolet Monte Carlo SS
5.7 LITRE / TES Headers / SShaker Hood / WC T-5 5-Speed1970 Pontiac GTO
Orbit Orange Judge Re-creation / YZ RAIII 400 / M21 4-Speed