hey guys,
i was curious what is the best tool to use?

scenario.

the inner lining is from an audi.
these cars have hard plastic linings. i need to trim some of it out. about a 10 inch x 7 inch area. round off the corners. make it look clean and possibly OEM. looks is my thing. if i can make it look like it was from the factory that would be a win-win for me.
perferably the tool will allow me to cut it while its still in the car. ill remove it if i need to. its more work to remove it so i prefer not to but its not off the table.

requirements.
* a tool that will allow a clean cut. rounded edges type of cut.
* won't destroy my hand muscles in the process.
* clean cut without jagged edges.
* power hand tool preferred.
* can be used in tight spaces. aka, under the wheel well area.
* wont break the bank.

so what tools do you guys recommend to trim this plastic liner?

So fender liners are usually some type of thermoplastic which play well into getting the look you want. It's not so much the tool you use but the series of cuts you make and how you sand the final cut. A body saw (electric or air) will work, a Dremel style tool with a cut off style wheel or even carbide burr bit, or even just a "hot wire" kind of cutter will make the initial cut inside/outside your line THEN use a powered sander on a medium setting to kind of slowly trim/melt your finished edges. And of course, I would not be trying to reach up in the fender well to do this. Instead mark and layout your cutting, pull the fender well out and do it on a bench. This is how we do it with kit cars and the kit trucks we build. We have to mod almost all the fender wells in these vehicles....