This happened to me. Turns out that I had water damage on all four speakers...two rear ones died and front two were intermittent, with only tweeters working.

Try hooking up a speaker you know works and seeing what happens....I had to replace all of mine because of water damage. Pretty easy process. If you replace the speakers, be sure to waterproof them.

 

Re Speakers cutting out



I had the same problem when I bought my 2002 Trailblazer...apparently rain water seeps in through the door seals and damages the factory speakers. I replaced all four of mine and they work fine now...not an easy fix, labor intensive.

 

I had the same problem when I bought my 2002 Trailblazer...apparently rain water seeps in through the door seals and damages the factory speakers. I replaced all four of mine and they work fine now...not an easy fix, labor intensive.

That's exactly the problem everyone seems to have with our trucks. Unfortunate because it is such an awesome model...

The installation isn't that difficult, but I spent A LOT of time waterproofing the heck out of the speakers with a couple baffles and some homemade housings for them....