1990 1500 Blazer... oil pressure gauge pegs when engine running but not when key in on position with engine off. This occurs regardless if gauge is hooked up to sending unit or not?? I will add that this Blazer was a police vehicle and the wiring harness is brittle. Could some wires melted together? Any help is greatly appreciated.

Sounds like your sender wire has a short between the sender and the gauge to ground.