I have a 99 Cherokee and when I bought it the back fenders were not on but in the back seat the metal pieces are still on the side of vehicle and there little plastic clips on the back of the fender I can't seem to get them to pop in or is there any easier ways to get them back on? Or could I take the metal pieces off the sides of vehicle and just use self tapping metal screws?

 

Here's photo

 

You WILL have to remove the metal brackets to put the plastic flare back on. When you remove the bracket you will end up snapping a few if not all of the studs on them. The best and easiest way to repair them would to grind off the old studs and replace them with some stainless steel nuts and bolts. and plenty of anti-seize when putting them back together.