I am going to be installing some stereo equipment on the floor directly below the back seat in my 99 Tahoe.

Before I start screwing into the carpet and whatever is below (screws are about 3/4"), can anyone tell me what is actually immediately under that carpet? Thanks

Always check for yourself. Something could be routed differently on this one, yours may have an option nobody else has, or someone may have installed an aftermarket part underneath.