Tried the goo gone and works good on some of the discoloration for newer stains.
When I purchased my 87 TW it was a fixxer and all the white plastic had a dull grey coating all over the surface. It was like a crusty layer of nasty discoloration where the plastic basically broke down. My solution was to take some steel wool to it. I first used a medium grade steel wool to remove the outer layer. I then went back over the total area with a finer grade of steel wool to smooth out the circular pattern of small scratches left behind. This final "buffing" smoothed the area back into a smooth bright white finish.
I am unsure of how it would work on colored plastics. I found a product after I renewed my plastic that I would have bought before my DIY fix with the steel wool.
This product looks pretty impressive and I included the link below. It is called PC Racing Plastic Renew.
If anyone gives this product a try please post a review so we can know if it really works or not.