do i need one of these things for an aftermarket deck? what do they really do?

This is the wiring harness that lets you put in an aftermarket head unit without having to cut the factory harness off to connect the wires.....this piece will also tell you what wires go where removing the guess work for the untrained audio installer....do you need it?? nope, but for $10 it makes life a whole lot easier

He's right...just connect the wires from the back of the harness to the back of the head unit. Then just plug in to the factory harness.