Have you tried putting a wire down the drains to unclog them?

I used a piece of high temperature stranded wire - it is stiff, but flexible to get down the drain. I bent the wire back on to itself about 1/2 inch so that the leading edge is smooth - I bound the fold with a small piece of electrical tape.

Once I unclogged the drains and after that, about once a month I poured a cup of water down each drain to see if it came out the bottom.

I had particular problems with the trunk drains in my '90 544 sedan. I removed the panel in the trunk and found that the hose ended in a plastic fitting that was too small for the hose and it was folded to jam in the fitting - this caused it to clog on a regular schedule. I kept the wire in the trunk
and cleared the drain whenever it did not pass the cup of water test.

The upholstry coming apart seems to be a real adventure - fortunately I never had the problem, not even pn our 17 year old '96 855R.

Have you looked at volvospeed, youtube, or pintrest?