Solvents like kerosene and WD40 (which I think is kerosene-based) are very gentle on just about everything (even paint). You have to wait longer for them to work, though. Wet a cloth with them and let sit on the dirty surface for an hour or two, or even overnight, if you have the patience.
I'm getting to like Gunk "Engine Bright" degreaser a lot. It'* a water wash off and very gentle on just about everything. Again, some patience is required. (nothing cleans faster than carb and brake cleaner, and I use them a lot, but they can be really tough on certain non-metallics. If you know your plastic is linear polyethylene or polypropylene, then these kind of cleaners are fine).
1998 Bonneville SE 75K
1994 Bonneville SLE 125K
(1990 Bonneville LE-ST (ST=Stolen&Totaled--sigh!)