I had a pickup that had a 10+ year old bed liner in it. Naturally it looked faded and full of dirt, it just looked like crap. I was in Houston some years ago with some heavy equipment I dragged down there for hurricane cleanup and ended with a bucket of hydraulic fluid spilled on the bed. I used purple power and a scrub brush to clean it up, ended up doing the bed sides too…the bed liner came out looking brand new, the black restored looking fresh like it was just sprayed. That’s a long way to say, purple power and a scrub brush, maybe even some hydraulic fluid first