Easiest is to use PB or equivalent and just spray the entire length of each belt trim piece, run the straw across the top edge, spray and let gravity do the rest. After all, this is how water and dirt created the problem.
how I did on my E9 I cut all the rusty bolts with this tools on the picture the blades are tiny but you will destroy the paint bot not that bad in my case I was going to paint the car so I care less about the paint but I did not damage the moldings that is hard to fined after taking out I spray easy off and remove the anodize that was badly scratch and use sand and German maid metal polish and now shine better than before after make it shine I spray special aluminum clear coting from CERAKOTE that is stronger than anodize and better shine but I tell you it took me 2 week to restore it it wasn't easy. I can explain more if you have more questions.I tried searching using a variety of terms and I haven't be able to find anything on how to remove the side trim.
If anybody has done a write up on this I would appreciate being pointed in the right direction, thanks.