I carried out this retrofit nearly six years ago and posted information for this (look under my id). In short, for the front seats you need to

(a) swap seats and plug in existing connectors to the car
(b) install the small control panel with switches (just above the front ashtray) to one with the switches for front seat heating.
(c) The passenger seat will work with further changes. The driver's seat will require either a jumper wire (an Austrian fellow posted this, based on his experiences), which I did to get the driver seat working.

The correct way is a add the relay that factory power seat cars have. It is located in the relay cluster of the FRONT fuse box. Part number is 61.35.8.365.960 (black, 5 pin).

You MIGHT need to add the fuse (front fuse box) for the seats, check your car. I didn't need to do so, even with a car with mechanical cloth seats.

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2002 Euro X5 (E53) 3.0 diesel (M57)
5 speed manual with UUC short shifter.
Pre-xDrive NV125 transfer case (chain & gears).
Factory Xenon headlights, Spyder LED tail lights
Updated vortex crankcase breather.
Motorsport dead pedal, factory fire extinguisher, factory first aid kit, factory F&R mud flaps, factory PDC

Bilstein B6 shocks F&R
Redline oil has replaced "lifetime" fluids (F&R diff, manual transmission, transfer case and P/S).
Dimple magnetic drain plugs in all compartments.