Technic makes harnesses for the three different systems and can tell
you how to run the wires and hook it up. Getting his harness means no cutting on your factory wire harness but you will have to split the undeseat wiring from the door wiring as they are in parallel in the base system. I have seen people talk about this on threads here but I don't know the specifics
there is an example in the above thread
Its possible to make your own I suppose but can you make a good one.