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At the risk of sounding like a nut who recommends the same fix for every problem, if it?s plastic this may be another job for a heat gun.

Use it with care until you get a feel for how the surface reacts and how far to hold the gun off the surface. I used a heat gun to rejuvenate the body side molding and bumper rub strips on a Saab wagon we used to have. Made it look like new and didn?t hurt the adjacent paint.

It can be used on many plastic surfaces that have faded or oxidized, to a point. Another option is to paint the piece with a paint meant for plastic trim. If this is for your all-original barn rescue car you may not want to paint it.

If the cover is metal, it could also be painted, with the same reservation about originality.