Is there a good solvent to remove the old adhesive or does one just scrape it off? Also, is there a trick to removing the hard plastic covering the lowest edge near the bumper?

Wow, digging up an old thread I see.

I did not use a solvent, I just used a plastic putty knife to remove the worst of it. Then I decided it wasn't worth it and glued the new one in. As far as I know it has no problems.

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Current
2006 E320 CDI, 57K bought at 67K "Liesl"
1986 190E 2.3-16, 198K bought at 56K "Brigitta"
1987 300TD (Chuggin Along), 292K "Friedrich"
1995 E320 Wagon, 200K "Louisa"

Past
1987 300SDL, (sold) 125K, bought for 1$ "Kurt"

used a glue or sealant to replace any of my door or window seals. The MB design permits them to be rolled into place and snapped into the channels. You will see what I mean after you get your new seals and pull the old ones out.

Just clean out under the old seal as best you can. Does not have to be perfect. I have done this on trunk (and window) seals on my W123 and 560SL with excellent outcomes.