GM dealers outsource stereo work to another company, which comes into the dealer to work on the product. I know that's for warranty work, so perhaps the techs were just too lazy to actually LOOK for themselves, they more than likely just phoned this other company and they said it wasn't possible just to get the GM guy off the phone. Bad information spreads real easy these days. Even at $100+/hr labour, you'd think they'd at least tell you that they can try.
'11 Ram SXT 4x4 quad cab - 5.7L V8; 128,200kms
'09 Malibu 2Lt 3.6L - 140,000kms purchased 1 March 2019
'06 Pontiac G6 GT - 3.5L V6; 162,000kms (rear ended 15 Feb 2019, and given totalled title by insurance)