I'm assuming that it's a guitar your picking up radio from, not a mic or other device.
Your guitars probably got a wire disconnected or missing, open up the electronics compartment(s) and look for a disconnected wire (usually from a volume pot to the bridge), or a solder pile with no wire in it.
Solder the wire back on, or replace the wire if it's not there.
There's also the possibility that the electronics compartment isn't sealed (shielded electronically). The inside of the compartment(s) and cover(s) should be painted with a black gritty looking copper paint or covered in foil. If not, I suggest buying a book on guitar repair (if your a do-it-yourselfer) and read up.
A very good book is "Guitar Player Repair Guide by Dan Erlewine" It covers everything from intonation to fret work. If your a serious player (who's broke like the rest of us), you should really have that book if you have any handy skills at all.
It could be your cable also, but if it's a used or cheap (or both)guitar it's probably the guitar. If the cable came in the case with a used guitar, it's time to get a new cable also.
Well that's as far as I'm going with this, Buy your own book

Torp