Annoying problem

Hey guys - Wasn't sure where to post this, so I'll try here, first.

I did a routine install in a 92 Geo Tracker today, no problems, tested fine, completed.  When my customer came to pick it up, there was no audio output, but the head unit was powered up, and the track was running.  I pulled the deck and checked my harness, which looked fine.  Tried her old deck in there, and it works fine.  I put a MM on every wire in the harness, ensureing proper continuity (sp?), and plugged back into the new head unit, still no audio output.  I got a new identical unit to try, and got the same results. I apologized, and sent her on her way as she had to be somewhere, but as she was pulling out, the output came back on.  She called me about a hour later, saying it comes and goes as it pleases.  I asked her to leave it off till tomorrow.

I dont want to waste any time with this, but I haven't a clue where to start.  The only thing I can think of to try is to tin the connections on the harness where they plug into the factory wiring.  If that doesn't work, I'm not really sure what to check next. 

Any help would be greatly appreciated

Thanks in advance

think i know the issue..... new decks are very sensitive, most likely there is a speaker terminal grounded to chassis.  This will cause the amp in the deck to go into protection and the rest of the speakers will shut off..... i would check all the speaker wirtes to see if any are grounded, then troubleshoot them on a per speaker basis.  seen it a million times, the factory deck doesn't have this feature so it will appear normal.... it comes on everyonece in a while while shes driving becasue that ground short is vibrating in the door or something....

Sorry, I shoudl have specified that there is no factory or aftermarket amp in this vehicle.  It's straight to the speakers

The grounding of a speaker wire sounds like the most sense.  I'll probably start there and see if I can get her fixed up.  I'll keep you posted.

Thanks for the advice