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If you look at aftermarket rear view mirrors with compass displays, you'll find they run anywhere from $200-350. apiece. Even though Lexus would pay less for them wholesale, I somehow don't think they would install an extra-cost item in tens of thousands of cars and just not hook it up.
I've looked at my mirror at all different angles with a flashlight and I don't see anything. Maybe '08's have a different mirror. But if it IS the same one, I'd think it's more likely they just use the same shell as the compass mirrors, and the "guts" for the compass display aren't in there.
I'd be happy for someone to prove me wrong, though ...