It does not auto dim, it just remembers the settings. As soon as it has a GPS fix and knows the correct time it will switch to either day or night mode, and use the last setting you chose for that mode. Just give it a minute or so to get that GPS fix.
Mapsource, you mean the one on the PC? That you can upgrade by downloading it from Garmin, If you mean the maps on the GPS, you gave to order the DVD from Garmin and pay for the unlock key.
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