There is a guy on EBay that restores and sells GM Radios .... I was able to find my RPO Code UM6 radio that came in my 85 Blazer.
In my restoration, I installed Kicker 3.5 low profile front dash speakers and Kicker 4 X 10 Rear Kicker speakers ... It definitely has old school sound and works fine.
I then found and installed a unit from Out Of Sight Audio using wiring harness adapters from Crutchfield ... One Inlet from Speakers and one Outlet from Speakers . I used the loose wiring from the Pigtails to wire into the unit.
I play Pandora through mine and it sounds fantastic. Any playlist should work ..
You can switch back and forth between "Original" and then the Bluetooth through your phone. I bought the MKIV so I could have hands free driving, I just have not used it very much based on the voice in is a mic that you have to mount.
outofsightaudio.com
Let me know if you need pictures or any help.
Thanks !