Mild heat will help take mild scratching out of some plastics but can alter textured finishes, if your going to try it, get some plastic from a scrap yard and experiment a lot.
if it’s really faded, vinyl paint works well. Needs to be mega clean. I’ve done plenty of mini door pockets and it’s good but like all paint it can scratch off and might need the odd touch up. In our well used runabout mini, we’ve a set of brown metro mk1 door pockets that have a speaker grill. Only ones I could get were brown, so wee black spray job and 3 years later they’re still good except where a passenger rubbed a high heel over a bit. There’s plenty of other paint options if you use the proper vinyl primer.
for mildly faded, go for some of the heavier silicone based stuff mentioned already. I like the autoglym bumper gel, though it does need a good buff off afterward and a bit of time to harder nicely.
Quick job - back to black spray. If you wipe it before it sets it isnt just quite as mega show shiny and can help even it out. The aerosol spray has a habit of coating loads inside a car unintentionally, I would spray it onto a polishing sponge and wipe on first if that’s a problem. I’ve been yelled at before for making the brake pedal too slippy in my wife’s car, ooops, it was accidental overspray, honestly.
there are some heavy duty vinyl restoration products out there but they can be expensive. I did use a good one by Ctechniq on an R56 mini exterior and the results were good at the time, though it took a lot of meticulous surface prep.