I picked up a neat electical troubleshooting tool recently that is kind of pricey ($80) but saves a lot of time tracking down problems. It is called a Power Probe. It is hooked up to a battery and has a 20' cord. Depending on which way you press the switch, you can introduce a power (+) or ground (-) into a circuit. It has a resetable 15A breaker so if you put 12V+ into a dead short, nothing is hurt. It also has a small lead that detects continuity, so you can test a fuse to see if it is good. I think you could make something that would do this job for less money, but it probably wouldn't be in such a small, compact package.

Remember 9-11-01--God Bless the USA
JUSTICE, not REVENGE, will prevail

1951 Chevy 1/2-ton Pickup truck