Thanks all of you for the good advice. Unfortunately I destroyed the trim. It just splintered under the mallet after I got the first few inches started. I used heat but not enough, possibly plastic that had just hardened with age, I don't know. If I were to have a second chance I would use a heat gun.

1963 BSA Rocket Goldstar, 1969 Triumph Tiger, 1969 Norton Commando, 2017 Yamaha FZ07, 1955 Ford Thunderbird