I know my 03 Eddie Bauer has an option that when you put it in reverse the side mirrors automatically adjust down so you can see better when backing. Possibly this is why. Just a guess. Haven't had a need to check out wiring.
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I don't believe they offered a backup camera option in 08, that started in 09. My rear view mirror auto dims when headlights are behind you, however, that feature turns off when you put it in reverse so you can see better, and that's the reason for a reverse wire at the mirror
Which wire is the reverse wire?