Make sure it's not a brake light. There's a slight delay before the sensor warns of a lamp out and since you have to press the brake to put the car in reverse, that delay is just enough to fake you into thinking that it's the back up lights. When you start the car, try just pressing the brake and holding it for about five seconds before shifting into reverse.

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250