[monkeyracer]

you could use the original harness if you want a nice clean install, but it might be a lot of work.
You would have to pop the pins out without damaging them, desolder, then solder the new wires and pop them back in.
The other option that would be a whole lot easier would be to just wire the new wires directly to the oem ones, and just lose the harness altogether.
If you want to be able to go back to stock easily, you could clip the wires and harness from the old mirror and wire the new mirror to the clipped harness, and when you sell the car, or to return to stock, just buy a set of new mirrors, or re-wire the harness. (This way the only wires clipped are the ones in between the mirror and harness.)

Post some pics if you can of the aftermarket mirrors and what wires and brackets are included. BTW those are the exact ones I want to get, but I want to paint them body color instead of leaving them black...