its either a bad bulb or incorrectly installed bulb or the wrong bulb in the tailight.
i've seen it numerous times,backfeeds through the circuit causing odd problems.

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location:rhode island, usa
1993 range rover classic county 2" lift, armoured, bridgestone at's.
1994 disco 1 modded out. ome3" lift,armoured trxus tires,

2006 subaru outback 2.5i
rirovers@yahoo/groups.com