I don't know...i own my 850 for 24 years and it's ac system(ECC)always does that after turning the engine off...for a few seconds.

Last year i replaced drier,evaporator and heater core while i was in that area...and it acts the same.

For me...it's normal behavior.

Aftermarket radio should have euro connectors...so you couldn't fry it.
Even if it doesn't have them...it have a fuse which would blow on short.

I would re-visit that throttle cable,especially if it is aftermarket.

Fuse ok?

If it's not contaminated...leave it.
It's not hard job...but scraping the old gasket is a pain because of bad access.
Don't scrap it with a razor blade or metal scraper.
Use plastic scraper since you don't want to leave scars on mating surface.

Probably the serpentine belt tensioner roller...or the belt itself.

I had evaporator raplacement job last year. What a job! I bought Nissens evaporator,NRF dryer and,while i'm in that area,heater core(NRF). Two orings for heater core was ordered from Volvo(around 20€). Total cost for parts only was around 280€ + professional ac shop review and vacuuming & fillin...

On these cars...in fact in almost every car ...if ambient temperature sensor is not working...ac will not start because ecc will not know what is ambient temperature in order to protect compressor...since ac will not work if ambient temperature is below 2C(when orange light for snow flake lights up)...

Read this:

No....on the passenger's side....where evaporator pipes goes through firewall....there is a pressostat with 2pin connector... on thicker(low pressure) line.
High pressure line is behind passenger's side headlight and it's a 3 pin connector and switch.