flipdown dvd issues w/ factroy stereo

Hoping someone else out there has heard of this and has a solution...

2009 Yukon with aftermarket overhead DVD and built-in wireless FM transmitter...customer complains since DVD was installed they lose the audio to their rear speakers.  Audio from rear speakers does not go away entirely, just diminishes appreciably.  Will always happen within 30 minutes of driving vehicle...then rear audio volume will start to come and go...frequency and length varies.   

This would seem to be impossible as there is no direct connection from the DVD to the factory radio...installation is hot, ground and domelights...DVD's audio is played through factory speakers via the FM transmitter. 

When the overhead DVD is disconnected problem disappears.

The customer and service director at the GM dealership swears to this happening.  Not sure about the customer, but confident the service director is not pulling my leg.

Anyone ever heard of this?  This is the 2nd time I've run across...the first time (a number of years ago) was a friend, told him he was crazy and never heard any more...his car was a Yukon as well, but definitely an older model.  Any opinions, thoughts, ideas on this mystery would be greatly appreciated.  Thanks.