Bought an 07 last week. PO took off his plates. Even though they don't look rusty, can t get the bolts to budge. The philips heads started to strip, so I stopped. None of my stripped screw devices fit. There are no bolts on the front, I am hoping he didn't force the wrong screws in. Guess I am going to a shop tomorrow. Any advice?

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I had a similar issue with my front plate actually, rears were fine.
I ended up taking a little PB blaster and used parrots beaks as the philips screwdriver was rounding out the screw center. broke it loose and put fresh stainless screws in there instead, should hold up ok.

 

PB, Kroil, Liquid Wrench like others have said THEN....... Try some GENTLE taps to the screws along with some LOW HEAT from a hair dryer while you are working on the screws.
Stainless Steel is a must for ALL non metallic fasteners exposed to the elements.

 

It just took a large phillips head and a lot of patient elbow grease to slowly back them out. the PO had the same plates on for many years, so while the bolts didn't look rusty on the outside, down the threads they did look they were rust bonded. I replaced them and put anti seize on the new bolts. They did turn in hard. Also the local auto parts store only had slightly longer ones, I noticed the fronts ones but up against the bumper behind the license plate holder, I may search for shorter ones.

 

Try Home Cheapo or Belowes for some SS nuts and bolts..... Both stores have a good selection.