99 Yukon Factory Amp

99 Yukon with factory am/fm/cassette and external (factory) cd player installed below radio. Aftermarket cd changer also installed using fm modulator to transfer sound. Radio will stop emitting sound sporadically for extended periods of time. The display on the radio stays on and the tape or cd continues to operate, but no sound. Dealer replaced radio but problem persists. On the 2nd trip to dealer friend was told that the problem was related to the aftermarket cd install. CD changer uses three wires (constant pwr, switched pwr and ground. Modulator hooks to antenna connection. I doubt this would cause the factory tape player to quit playing and I doubt it would cause the factory cd to quit or the am/fm radio to quit working. I suspect this radio has separate amp, but have not seen the truck to confirm. Can somebody confirm this for me? If separate amp, would probable location be behind glove box? My first guess is that if the lights remain on at the radio and the tape or cd continues to play, the problem could likely be at the external amp if so equipped. I hate to get in the middle of "friends" problems, but they are so hard to resist when they are cute!