My own 2˘:

Having known several people with cars that have built-in NAV ability, every one of them wound up using their cell phone or a standalone GPS device. I use a 2013 Garmin when I travel and it is far superior to most of the built-in GPS devices I've dealt with. The biggest challenge to any GPS device is how updated its map software is. At least with a detachable GPS or a smartphone, you can chunk it and get a new one when it is too obsolete to run the newest/latest/greatest revision. With a built-in system, you now have a fancy piece of panelling with movable pictures on it. Personally I wouldn't waste my time or money installing a built-in NAV system on a 13 year old car. How useful is your 13 year old cell phone, or 13 year old laptop? The GPS device will be similar in its antiquation.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)