Hey, Dan...how's it hangin'...?
I've not had any direct experience with removing the factory-installed liners, but I would suspect they're stuck on with a rubber-based contact glue...same stuff as Top of the Hill uses on their pre-cut padding. Most times, using something like a heat gun to heat and soften the adhesive works well to help loosen its grip. Then, once removed, use something like lacquer thinner or acetone to clean the rest of the glue off....watch the fumes and use protective rubber gloves...both of these will eat your hands UP! Also - DO NOT - repeat,
DO NOT - let either lacquer thinner or acetone get on your paint!!! I don't need to tell you what either one of these will do to your painted surfaces, do I...?
Then, wait for everything to dry....you'll still leave some glue behind, no doubt...wait overnight, if possible.
Then, it's just a peel and stick application after that.
I found that the stuff works best - for me, anyway - by peeling only about a half of the paper backing off, then position one of the edges of the piece where it needs to be...then stick it down and continue slowly peeling off the backing and sticking it down as you go. Be careful not to get an air bubble under it as it will pull loose from the inside of the fender later on....ask me how I know this....go ahead, just ask...
Also, I used a small wooden wall paper seam roller to roll down my liners...worked well.
Hope this helps...