83 Mitsu PU Diesel. Oil light on drawing .25 amps when key is off. Key on the light goes out. All other lights and gauges work as they should. Light is off when the engine is running. It is killing the battery when parked. Where would the idiot light get power when the key is off?
OP answered his own question in the comments but is not responding.
Placing the commented answer as THE answer.
Found the problem. The air cleaner housing is electrically isolated by being mounted on rubber bushings. Pinched beneath it and chaffed is the main alternator wire and the oil pressure wire. They were shorted together. The Alt wire is always hot and when the key is on the idiot light is a balance circuit and is not lit. Go figure!