An IFT. Found probably smoking in the head or at the galley vent or sleeping next to the P-chutes, waiting for the opportunity to yank a box out of the rack, smash it with a hammer or flashlight, reattach the plugs and kick it back into the rack. Then the gear works, just like magic...
We had been having problems with our one of our UHF radios while we were out in Sig and called our IFT to see if he could do anything. I had just got out of the seat and what do I see the IFT do? Pull the RT out and drop it from about chest high before sticking it back into the rack. Didn't have a problem with that radio again. I can think of no logical explanation for why that would work, but it did.