My seat only locks on the drivers side
 

and is loose on the passenger side, its a adjustable bench in a 72 c-10.............my stepdad said im missing a pin?

Where can I get this part?

Its more than likely that the wire that runs across to that pin under the seat is bent not allowing the pin to engage. Just look under the seat and see where that wire runs from the seat latch to the other side and you will see where it engages the right side rail. It doesn't take much for that not to engage.

If you do find you are missing the latch piece let me know I have one from an old set of seat brackets I am not needing. You can have it for FREEEEE!!!!!! It is about 3 inches long, has several notches cut in it and is about 1/4 " thick. Let me know if it is what you need. Fits into the seat bracket and hooks one seat bracket to the other seat bracket through the use of a wire.

If you stuff junk under your seat, then it is REAL easy to bend the wire that controls the pin.
If you can't figure it out by looking under the seat, pull the seat out (8 1/2 inch bolts) and flip it on its back. It is a very simple design.

on the passenger side there is also a spring and tab that keep the tooth shaped metal peice in place.... mine was missing this ... it would pop loose only when i had a passenger ..then there side of the seat would "float" back and forth while i was driving ...strange sensation .... i got the tab and spring at a boneyard for nothing