Hi.
Start with a really good clean with an "All Purpose Cleaner" using something like an old toothbrush to get them really clean.
Then use Gtechnic C4 and ideally cover the car/put it in a garage overnight to allow the coating to cure.
If you've never heard of Gtechniq, they are a small company based between Northampton & Daventry and make many excellent products:
https://www.gtechniq.co.uk/about-us/
https://www.gtechniq.co.uk/shop/auto...trim-restorer/
Hope that helps and Good Luck.