Based on what I have read, the power sent to the reverse light is in pulses, which may explain your flicker. You need to use a relay and pull power directly from the battery.

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2003 530i, Silver on Grey, BC Coilovers, Square 9.5" M-Parallels, Gloss Black Roof
2007 328i, Alpine White on Black, Sport/Nav/Premium
SOLD: 2006 X5 4.8is, Lemans Blue on Black