Three of the four speakers to my car stereo stopped working

I own a 2003 Subaru Baja. About 2 years ago, during a particularly wet period 3 of the 4 speakers to my radio stopped working. Once things dried out all speakers began to function normally...for awhile. Eventually those 3 speakers stopped producing sound altogether. However, this past summer, to my surprize on a few occasions (always during a spell of dry warm weather) they would begin to operate again, but only for a few hours or 1 or 2 days at the most. The only common denominator that I could rationalize was the radio itself, so I replaced it last week and those three speakers still aren't working. Before I spend more money replacing speakers or paying someone else to troubleshoot, what should be my next logical step in perhaps solving this mystery on my own?