Cage/bar behind drivers seat. I also have a factory hard back window, so the plate sits INSIDE the back window. It's lighted and everything to be street legal according to Utah laws (illuminated always, etc.). And...I might add...it was a real pain to get it mounted and wired and working there, but it was the only place I knew I wouldn't knock it off or it would interfere with anything. (like game baskets, spare tires, tailgate operation, etc.).
I agree with Tripplec...these manufacturers need to be aware (more aware) that a whole lot of these are requiring license plates and they should just include a feasable mounting area/location or even bracket somewhere. And not just hange a few wires from the back where the plate will surely get torn off, make a REAL location.
Done ranting.
Edit: I guess I'm not done ranting. The whole street legal kit needs to be added from the factory even if it adds a couple hundred to the price. I did mine myself because I hate the way other people hack and wire stuff in and it's done right, but it was done in NOVEMBER (of 2020 right after I bought it) in the freezing cold. Sure would have been nice to just buy it with the turn signals, horn, license plate mount, etc. already on there. And I wouldn't have given a flying fart about the extra money had it been installed from the factory. I likely paid more than that for the parts to do it myself.
Ok, now I'm done.