I was wondering if all trucks had standard wiring for things like power seats and other accessories or if each wiring harness was made for whatever that truck was equipped with. Will the harness be hidden under the carpet if I wanted to put in power seats in a truck without them??

 

Yes and no. It depends on year as to what may or may be there.
My 99xl Has the fuses and relays for Power seats, power locks and power windows. But the wires end in the kick panels.. So fare I have added power locks and seat. Just find the wires in the plugs and run them where needed.