What I noticed around here is that if a part from another car doesn't have the same part number as your x5 part they will tell u it won't fit. I'm pretty sure you can remove the seat rails from the e39 seats and swap them with your e53 rails. now for the wiring, i don't know but I'm sure you can cut and splice to get it to work but a lot of people on this site doesn't know anything about that