I have always used a power cable with all of the gps' ive used on Dirt Bikes or in Land Rovers...saves a lot of $$$ on batteries and a lot less hassle.
The GARMIN cigarrette lighter adapter cable is part no:
010-10085-00 $27.95 CAD
Thats at GPS Central in Calgary again, it's the 4 pin, works with lots of the later Garmin models.
Dont think my girlfriend has bought a mapsource yet - I will ask, though if you check out the Garmin website, it gives a copy of the detail available for a few of the different ones.
She was planning on getting one or more for a trip to Inuvik and Alaska in June we are doing.
Will let you know what happens