I've got power heated front seats in my XLS that originally had manual seats, so I think I qualify as an expert on this subject.
Edited to rearrange my thoughts a little and clarify.
First off, to make the seat work, you will need a fused electrical circuit, then the associated wiring to the seat, and that is just to give the seat power. Some vehicles have "memory position" for the seats too, which means that a "module" and associated wiring will also be needed. You could just "hot wire" your own wires from a power source with a fuse in the link for safety reasons.
The memory module can not be made to work in a car that doesn't have the supporting wiring and other modules, which would be a moot point, as a truck with the harness would have memory seats in it already. You'll have to bypass it coming from a memory truck into a non-memory truck.
This can be done, but it's an extra special adventure that frankly is only worth it if you just gotta have the heated seats, or are too cheap to go out and buy multiple sets of seats at the junkyard... I only went that far because I was pissed off and didn't want to admit defeat, and didn't want to make
another trip to the junkyard for a set of seats.
So the wiring thats already present under the seat in the 2003 (I'm assuming it has to do with seatbelt sensors or something?) won't be enough? If its just a matter of running a fused 12v line then I can probably do it (I CAN do it, but will I get around to doing it...), but I'm certainly not wanting to deal with any more work than that just for power seats. The old truck doesn't have the memory position buttons, so thats not an issue.
No, it probably won't be done using the wiring already present.
If you've got 2 plugs on your manual seat, look closely and see if there's a (relatively) fat pair of unused wires. Thats your +/- for seat motor assy, and if so, you're all set and it might just plug and play. I really strongly doubt this will be the case though.
There's more likely to be 3 plugs under your driver's seat on the manual-seat truck. Seat position sensor, seat belt buckle sensor and seat belt pretensioner lines. Your power seat will probably have 2 plugs, one for the seat belt pretensioner and then a separate one that houses the seat belt buckle switch and seat position sensor wires as well as the thicker power leads for the seat motor assembly.
Coming from a non-memory truck makes this easier. You'll want to swap the seat belt anchor point and the wiring with plugs over to the power seat, so that those two plugs match the car. You also want to do this with the seat position sensor and wiring harness. From there, just cut any wires off the seat that aren't needed anymore, and you're left with the two that run the power seat motor assy. Feed them from a new fused circuit and you've got a powered front seat.
Notable: once you have the circuit added, a power passenger seat is equally easy to install, just need to run a jumper harness under the center console. There were no memory passenger seats.
