The only Nav retrofit info I've seen is over on E90post.com. Somebody put a full iDrive dash with Nav into his 2006 E90. It was an extremely involved (i.e. expensive) project but seems to have been successful. I'd imagine an install on an E70 might be a bit easier since the dash does not need to be replaced. However, the car will still need a new screen and programming. I don't think those types of extensive mods are all that common on the E70 however.

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2012 X5: BSM/Black/Sport Activity/Bel STi-R Plus

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2013 M3: JZB/Black/ZCP/M-DCT
2010 Audi S4: Deep Sea Blue/Black/Prestige/S-Tronic/Full ADS
2006 BMW 330i/Jet Black/Black
2005 Audi S4/Moro Blue/Black