Newer cars have flush-mount glass but finished with a metal-backed rubber channel. I tried to peel this out, but the backing started to deform and I didn't want to have to replace it if it broke. However, you can't just mask this channel off, as it will leave a hard tape line at the transition between the channel and the roof. 3M sells the solution to this problem—trim tape. The exposed blue section is a semirigid plastic that you feed under the rubber trim.