I've looked up this same thing for the Jeep Grand Cherokee, as it also has the same plastic body cladding. A 50/50 mixture of boiled linseed oil and paint thinner applied with a rag seems to restore the plastic, but it only lasts for 4-6 months before needing another application. Be sure to take proper precautions with these chemicals, and be careful of the rags after use as they are combustible. It's actually advised to burn the rags after use if you can.
I recently bought my sister a 1996 Jeep Grand Cherokee Laredo, and I'd like to try this procedure for myself. I have a lot of other things to first fix on it though.
The wife just bought some trim restorer produced by Turtle Wax....Works well, reminds me of an exterior Armor All. Not sure how long it will last, but I suppose it will hold up at least 6 months at a time if kept up on.
best way to restore is to actually paint it. Thats what I did to the bumpers on my PT. you'll want Duplicolor Adhesion Promoter and Dulpicolor Bumper Coating (At least this is what I used) wash the areas to be painted with dish soap first then tape off so you don't get over spray, couple coats of Adhesion Promoter and then as many coats of the bumper coating as you see fit. (as I recall it was 2 coats of adhesion promoter, and 3 coats of bumper coating, but I painted them black, they have white or grey as well)
"Mothers Back to Black Exterior Bumper & Trim Care" "Revitalize & Beautify All Color Trim & Flat Blacks" <-- That is the whole front label, red plastic bottle. It comes out as a white, creamy looking lotion.
Directions for use are as follows.
"DO NOT APPLY TO HOT SURFACES"
1. Surface to be treated must be clean and dry and cool.
2. Pour Back to Black onto a dry, terry cloth, sponge or suitable applicator.
3. Apply liberally to surface. Massage product thoroughly into surface.
4. Allow to dry, then buff with a clean terry cloth or microfiber towel. Should any streaking occur, some additional brisk buffing will be required.
5. Heavily stained or soiled areas may require an additional application.
My front splash guards on my 06 Grand Caravan were grey and dingy looking. I applied Mothers Back to Black only one time, polished, and they are beautiful, I did the rest of the black plastic and it looks really nice.