Cleaning "Goop" Off TPG Holders

I use Slab-Renew. It takes the scratches and goop/stickers off the slab, makes it look like new. It goes on and dries clear, really like how it gets rid of those annoying slab scratches.

As noted already headlight cleaning solutions are in almost all auto parts stores and in the auto sections of Walmart, Kmart, Sears, Target, etc. There are many different kinds and oddly enough I've had great luck with one of the cheapest versions. At Walmart I've seen about 5 or more different ones with prices from a few dollars to as high as $25.
I've used it on almost every type of plastic with no ill effects.
Don't know if you use those outdoor Solar lights but one of the biggest problems with them is the collectors get all dirty and messed up so I use that stuff on them and they look new again. Not only takes off discolorations but keeps dirt off really nice too.