I recently purchased a 68 RT charger that didnt have a build sheet in any of the seats or above glove box.

Any suggestions on other locations to look?

Thanks

They did not all come with them. They were only stuffed in cars to cut down on trash buildup on the assembly like. There was no company policy to specifically place them in the cars.
There have been instances of them being under the carpet, above the headliner, pretty much anywhere they could hide them from view.

I agree if you don't find one in the seat springs, then under the carpet is your next best place to look .

Greg