Could be corrosion on the connector. Sounds like it might possibly be a wire with a break. While it is in a stationary position it has continuity and current passes thru to the bulb. If it moves a little, the wire strands separate and current stops passing through to the bulb.

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Jay Yambrovich
Scotts Valley, CA.

1993 300 CE Cabriolet (A124) 131K miles
1997 C-280 133K miles

2000 BMW R1100RT 69K miles
1989 300 E 216K miles (sold)