Those screws rust themselves in there after years of that Chicago weather no doubt!
I had the exact same problem the day the buyer of my 03 came to pick up the car...I sprayed a ton of that Toyota lubricant for frozen bolts on and around the screws, let it set, and finally worked the screw off so I could remove the plates and seal the deal!! After a couple turns to separate the screw head from the plate, I used a vice grip to expedite the process. The screw obviously needed to be replaced after all was said and done. Good luck!