The screws actually thread into an insert welded to the backside of the sheetmetal. Liquid wrench or pbblaster is about all you can do.
My old screws were plastic. The head broke off one while closing the trunk. The other head broke off while trying to remove it. The rust on the trunk threads froze the plastic screw.
After I removed the screws, I re-tapped the holes (M6 metric tap). I then replaced the screws with stainless steel screws. I also used lock-tite. Lock-tite should help keep the threads clean and avoid seizing in the future.
I tried plastic screws as my initial replacement, but they fit poorly (very loose). This would allow corrosion to penetrate and seize them again.
I had a little foresight several years ago -- and bought stainless tamper-resistant license-plate bolts to start with. The stainless part helps to prevent seizing. The tamper-resistant part helps to prevent license-plate theft. The kit of 4 was about $10 at the local auto parts store, IIRC, in a display right next to the license-plate frames. Just make sure you get the metric 6mm size.
I also put some foam weatherstrip around the perimeter of the plate, between the license plate and the car. The kind you use on house doors, with adhesive on one side. The foam strip prevents the license plate from ever vibrating, which might make noise or scratch the paint. Be sure to leave a gap at bottom so any water that gets in can find its way out.
I've done this for each of the last several vehicles... no problem so far.
you could use blue loctite, they are for smaller bolts (they come in different colors and applications) The red loctite are the ones that require heat and muscle to loosen (they are permanent) I use blue loctite to secure the adjustment bolt for the chain on my garage door opener and it's not that difficult to remove them (I just replaced my garage door motor a few days ago)
Okay. Recently my state issued me new license plates. Until now, I just got new stickers when I renewed my plates. So I couldn't kick this can down the road any longer.
I tried to remove the pan head screws that held on the license plate, but the one still wouldn't budge. I used my Dremel tool to re-slot the head, and went to the hardware store to get the biggest flat head screw driver available. Also tried to spray in some penetrating oil and let is sit, but no success. NOTHING would get it to budge. I tried using my Grabit screw remover with my power drill, and it couldn't torque them out either.
My final option was to drill out the screw. I did so VERY carefully, making sure not to mess up the threads in the deck lid. Turns out I was lucky. The screw finally did drill out in pieces, and I had to remove the inner deck lid fabric liner to remove the part that was stuck on the inside. It was extremely corroded -- probably from salt spraying up on the back of the vehicle over the past few winters.
After all this work I went to the hardware store. The size you need (and I couldn't find this ANYWHERE on the internet) is 6 mm x 16 mm 1.0 pitch. I got stainless steel ones for 62 cents each from my local hardware store.
It is maddening that Toyota didn't provide stainless steel screws in the first place. I had to spend hours dinking around with this, making multiple trips to hardware store, etc. I can just imagine some middle manager doing a Powerpoint on how they could save a million dollars by buying a cheaper fastener for this application by saving 10 cents per screw per car!!!
Lesson learned: never use regular screws to fasten on your license plate. Get stainless steel screws now, before it is too late. If my experience can save just one person from the aggravation I have been through, I will be glad this helped.