Door speaker

[thunderstruck]

It is possible you have a broken wire in the door jamb rubber boot. But its far more likely that you have a bad speaker. The GM speakers are notorious for the wire breaking on th speaker that goes from the speaker terminal to the cone. Sometimes it will touch and make the connection and work for a while. But then you hit a bump and the speaker stops working.
And the front doors use a 6.5 inch speaker or a 6.75 inch. The rear doors have 5.25 inch speakers if you have a crew cab. If its not a crew cab they have 4x6 speakers in the rear.

Agreed. Just about everyone I know of that has an '03-'06 GMT800 truck without Bose has had at least one speaker fail by now in the exact same fashion.

If you want to be sure, you can swap the speakers from left to right.