I've got screws in my lower plate holes on the rear

 

Those plastic inserts come in a variety of sizes. Any competent counterman should be able to look up the correct size for your vehicle in the Dorman catalog and order them for you. If all else fails you could use a screw with a nut threaded onto it from behind.

 

There isn't any room to get your hand or a tool to the back side of the bumper to use a threaded nut on the backside. You'd have to take the bumper off.

I'm thinking they cut the square holes too big since none of the plastic inserts from any of the parts stores work. I doubt they make a plastic insert to fit in a 1/2" square hole as all the others are alot smaller than that.

 

If memory serves me correctly, my 2004 didn't have the captive nuts in the bottom holes. I added them myself. Is it possible the previous owner measured wrong, and the fact the holes are too big is a screw up? Maybe the captive nuts for a truck are bigger? Just a guess.

If you're okay with having only the top two holes securing your plate, you could get a set of stainless or chrome licence plate screws that include matching hex nuts. That gives the look of four screws, but only two are actually functional.

 

All I know is the square holes are 1/2". Which is too big for the nylon inserts from the store. The square holes should be closer to 1/4". Must have been a GM screw up.

I was needing the 4 screws for the mounting plate and lense I was going to use. I guess I'll figure something else out. I was hoping there was a special insert that was used that the dealer could order. But they think there isn't supposed to be screws for the bottom of the plate by looking at the parts catalog.

 

I know what you're talking about. I put a protective lens over my plates also. For the first couple years I used screws with standard hex nuts in the bottom holes of the plate to hold the bottom part of the lens secure to the plate. And two regular plate screws in the top holes to hold the plate and lens to the bumper. When you looked at it, it appeared as though all four screws were holding the plate onto the bumper. In reality only the top two were functional.

I added captive nuts for the bottom holes myself when one of the captive nuts in the top cracked on me. The dealership parts department gave me four of them free. I think I had to use a utility knife to cut the bottom holes myself. Not 100% on that though. It's been a while since I did it.

In your case I would simply get four matching screws that are long enough, and have a machine thread (threading that will accept a standard hex nut). Use the hex nuts on the bottom like I described above. Then screw the plate and lens to the bumper using just the top two screws. It'll be more than secure enough, and all four screws will match. I did exactly that with my rear plate on my 99 Grand Am. And never had a problem in the 8 years I owned it.

That way you won't have to keep searching for the captive nuts.