Lift the rear seat bottom out, remove the plastic trim from the door edge that holds the carpet down, then pull carpet forward to expose the welded on studs that hold the lamp housing. The wire was actually wound around the stud. Measure carefully, then cut the opening in the carpet. I cut it undersized first and enlarged it until it was just big enough.

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles