@look4judy - You don't name the manufacturer of the fenders, and maybe it doesn't matter. I was advised by Taylor to use Mineral Spirits and a heavy cloth or fine scrubby pad. They also advised to NOT USE ACETONE. The problem with acetone is that while it does remove dirt and marks, it also attacks the top surface of the vinyl and leaves it soft and slightly sticky. The fender will then soil much more quickly and will have a "sticky" feel to it requiring more frequent cleaning.
After trying nearly every cleaner I can lay my hands upon, I have found that Z-Tuff cleaner works the best, again using a fine scrubby. A close second to Z-Tuff is the LA Awesome All Purpose cleaner that is found at Dollar Stores. Lots of elbow grease is required; there is no free lunch.
I wrap toilet paper on the fenders and keep it moist with trisodium phosphate (TSP). You got to let it soak over night, scrub it off with stainless steel pots-and-pan scrub bud and you'll get it looking brand new. Never Acetone. It melts the top layer.