I did my heated rear seat retrofit a while back, easy!
the relay is the first one (light green) in the REAR fuse/relay compartment when you open the rear panel (where the subwoofer and gas cap release are)
I was lucky when I did mine, because I got seats that were the same color and material as mine.

I also did the PDC retrofit, the only un-nerving part was cutting holes in my bumper!
but they were marked out very well, and it was pretty simple to do!
running the wires was easy as well.

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2003 X5 3.0i (02/2003) sport package, Sapphire black metallic / Black Dakota Sport seats. compass mirror retrofit, trailer hitch retrofit, Prodigy P2 trailer brake controller installed, PDC retrofit, Lumbar support retrofit. heated rear seats retrofitted. Running boards retrofitted and subwoofer install in progress. 4.8is Brakes installed front and back. Bluetooth TCU installed, Rear Fog lights installed. Retrofitted reclining rear seat backs.