the only thing I can think that it might be is a bulb in the rear cluster that's only used for parking lights. there seems to be lots of different 'squares' in the lens back there. I've never check which one is actually on with the parking lights. Can't be the front as they're your 'running' lights and indicators too.

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1993 320TE M104
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past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
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"music and women I cannot but give way to, whatever my business" -Pepys