HI all,
I have a 2001 3.0l x5 and i had issues w/ my DS (and PS) rear light not working (one that stay on) last winter. I got a couple of new bulb holders and bulbs but still was not working. Then i took the bulb holder out and cleaned the surface were the bulb holder screws onto the tail light housing unit to make the connection. It worked but kept going on and off when i drove around. I took the bulb holder off and sprayed some contact cleaner (buy at radioshack and etc) on the plate were the bulb holded make the connection (bulb to turn on) and it worked. In the late spring the same bulb would not work so i sprayed contact cleaner again and have not had a issues seen. The housing unit plate were the bulb holder connects gets corroded easy.

Try contact cleaner.

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2001 X5 3.0i "Dodi"
1993 Range Rover County LWB (71,600 miles) "Bertha"
1991 Range Rover parting out