Six years of cop sweat, cop hair spray, cop hair gel...looks like a colony of bacteria that had fused into a mass of plastic.
Went out and got a Bissell, cleaned the driver’s seat then worked on the headrest. It took me all of three minutes to scrub it off.
All clean now. You may ignore the fact that it is sitting on a toilet. This was immediately after I had finished the cleaning, it was still damp.