Do you have a recent pic of the car before it went in? Does the dealership have cctv?
A well worded letter via signed for delivery to the dealer principal with a cut off date for rectification or you’ll go small claims court with your evidence
That’ll be a tight one but the dealers won’t Es t the hassle and if they don’t show you’ll win
The crux is you need a before and after really or it’s your word against theirs in my untrained eyes
Do you norm hand wash your car?
You could also do with an “expert witness” statement - perhaps a friendly body shop could comment for you in writing ?