Radio - Pioneer 3 - Front Speakers Not Working

Partners, need some sound system troubleshooting advice.
'92 525i, 240K miles.
The stock radio (Pioneer 3) system was the best sounding system I'd ever had.
Last year, the left front dash speaker (2" midrange?) started crackling. Then it went out completely. When that happened all the front left speakers went out.
I thought perhaps that one bad speaker would keep the others from working on that side. so I pulled the dash one from the right side to test in the left but still nothing. I put the right one back in and just continued listening to only the 3 other corners.
A couple months later, all on the right front side went out at the same time.
So I'm down to the rears with the right one starting to crackle.
I checked to see if the wires to the amp in the trunk were loose/pinched/cracked but all look good.
I'm look for some additional troubleshooting ideas to figure out if it's the head unit, amp or other component.
Like I said, it's the best sound system I've every had and I'd like to keep it.

2 questions though if I have to replace it:

- I think I can get a stock radio from a '93 525. I would have access to the vin of the car so would I be able to go to a dealer and get the code, and if so would it work in my car anyway?

- can I just buy a new radio (like a Pioneer DEH-X6500BT) and install it without having to do anything with the amp connections?

Thanks for the help, Miss the good sound in my car.
Steve

bump?