Pull all the breakers, energize one circuit at a time, verify that it is working correctly, then pull that breaker, and move on to another one.
Time consuming, yes, but by isolating things it gets easier to find the problem.
If you energize a breaker and the unit (radio or whatever) doesnt work, start at the breaker and check for voltage, then the other end of the wire, and so on. Dont forget to check grounds, use a jumper wire for a ground as a final check, if needed.
The above should find most problems, unless the issue is internal to the unit not working.
Good luck, take your time, dont rush things.
__________________
Mike Starkey
VAF 909
Rv-10, N210LM.
Flying as of 12/4/2010
Phase 1 done, 2/4/2011 
Sold after 240+ wonderful hours of flight.
"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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