'89 C1500...radio question

I just got this 89 C1500, it is showroom perfect (no joke). At 94k original miles and ice cold a/c, this truck is all power options and runs and drives just like it was just bought from the dealer in 1989.

Anyways, just wanted to give you a mental picture before anyone thought it was an old "piece" with issues.

The only issue it does have is the factory stereo....it has the head unit, eq w/ cass and main brain unit. When the radio is turned on, it crackles really loud through the speakers and I can hear it rather the volume is up or down. I can still hear the radio stations playing if I turn it up, but the crackling noise will not go away at low volumes. If I turn the radio off, the crackling stops. Question....is it more likely the head unit or the main brain unit?

I took the amp/main unit and head unit out today and blew canned air thru both and checked all wiring connections. All wires appear to be brand new condition and both head unit and amp/main unit also looked practically new. Plugged everything back in securely....to no avail, same issue. Crackles and pops constantly (pretty loud too) with the radio on.

With these units becoming more and more extinct, and purchasing used/old replacements with a 90% probability that they have issues of their own....does anyone know if these units can be sent in somewhere for a rebuild/repair?

I do NOT want an after market stereo!!

stopped at a wrecking yard today and picked up an amp/brain unit. It looked old n rusted up but it was cheap and worth a try. It worked, radio doesnt crackle or pop anymore. How long will it last, who knows but at least I know what the problem was.