It's very rare for two speakers to quit at the same time, but to check them, switch the two front or the two rear speakers side-to-side. That will determine if the problem is with the speakers or the radio.
GM radios have a very high failure rate. Unfortunately, they will not allow us to buy radio service manuals since 1994, so you're tied to the dealer and their grossly over-priced repair centers. The typical cost of removal, shipping, repair, installation, and programming by the dealer is around $450.00. To insure you don't just buy a high-quality aftermarket replacement, they are now building the Body Computer into the radio. Without that radio in the truck, you won't have cruise control, power windows, and other functions. It's also the master computer that tells the other computers to "wake up" when you turn on the ignition switch.
Can't think of any advantage for the owner to pulling this nonsense, but it makes a lot of money for GM. Sorry.