Wow, I guess this is a common problem! When I bought my 1987 560SEL the ashtray wood handle was sitting in the glove box. I scraped off most of the black foam with a razor blade and used some 2 ton epoxy to re-stick it. It's been almost 1 1/2 years, and it's still on there. I live in Northern California, so I don't get the SoCal heat, but we have had quite a few balmy days, and it's holding up just fine.
Justin