Harry

My daughter has just purchased a 99 F which had just received an engine transplant (both distributor type) due to catastrophic HGF.
About twenty minutes after taking delivery from enthusiast seller the oil pressure light came dimly on then came on stronger.
Car towed home. 
Sensor changed and same problem after about twenty minutes running around the block.
Removed sensor, fitted oil pressure gauge and monitored oil pressure recording minute by minute stationary at idle. Pressure was fine starting around 78psi and falling to 21 psi as oil warmed up. This took about thirty minutes.
Oil pressure light came on as dull and then increased in intensity after about twenty minutes running at idle. THE OIL WARNING SENSOR WIRE WAS NOT CONNECTED TO ANYTHING AT THIS POINT!
The only thing like this I can find on the Forum is David Aiketgate's report about installing matching oil temp and pressure gauges where as a consequence of using the original pressure warning light wiring and splicing in a feed to his new gauge the original lamp glowed dimly. Obviously it was reading an earth through the new gauge. David removed the lamp as a solution. 
Can anyone give me an idea where to look for this errant earth or do I just run a new wire direct to the lamp?
Is it possible the engine loom changed? I haven't checked continuity between the switch and the lamp.
Any thoughts greatly appreciated

Brian