My first steps would be to disconnect anything to do with the radio. Then go after the blower motor problem and put it back to the way God <and GM> intended.
The idea is to undo anything that was done to the stock system, then trouble shoot the original problem. Hopefully you find 1 major problem that is causing many symptoms.
I find the turn signals flickering the DIC interesting.
WEAK OLD BATTERY? CORRODED GROUND POINT. These cars have.... grounding issues, to put it mildly. Start simple, the battery. Dirty/loose cables, grounds to the frame, to the engine. There are "Harness" ground points for lack of a better term. Under the hood, under the dash, under the carpet. Check for water damage. <flood car?> Wet carpet? The sunroof drains have a tendency to fall off inside the roof and dump water into the pass compartment. Is it all dry in there? Also the AC drain clogs and fills the blower with water then drips out into the passenger front footwell.
There are a ton of resources on here including all the grounding locations in a PDF somewhere...
