Just bought a '94 pickup, and among other issues from the previous owner, my reverse lights are always on. Before I get under there and start digging around, are there any usual causes for this?

 

Check the plug on the side of the transmission. Sometimes the wires short out against each other which makes the truck think it's in reverse.

 

Or the reverse switch may be stuck on. Simple test is to unplug the connector there, like noted above, and if the lights go off, then the wiring is fine but the switch may be bad.

 

Changing the switch is very simple: unplug the connector then unscrew the switch and put a new one in. You need a 27mm socket or wrench, or a large adjustable wrench. It shouldn't be too tight.

The switch is on the passenger side of the transmission, fairly high. It's the only electrical component in that area.