Most solvents will harm paint eventually. Your idea to try solvents somewhere inconspicuous is good. Alcohol is probably least damaging. Acetone and the various ketones, etc., are worse. If you pick the right one, they will also harm rubber and plastic. If the glue or sealer used by the PO is silicone-based, you may have no choice other than to scrape it off, a little at a time. I have never found anything that will dissolve silicone glues and sealers.

While you have the taillight lenses out of the car, you might want to check them for leaks. I say this because my 123 car came to me with a couple of leaks where the amber plastic met the red plastic, etc., in the lens. In one place, it looked like the lens had been 'tapped' in a parking lot just enough to produce a crack. It wasn't even wide enough to get a knife blade into but water could get in. I smeared -- wait for it! -- silicone seal into the crack and stopped the leak. With the lenses off the car, you can simply fill them with water and watch to see if any drips out. Since I could not get my lenses out (glued in like yours?), I gently sprayed water onto the outside of my lenses and watched inside the trunk.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970