I was going through the same thing a few months ago and finally decided to buy a portable unit instead. I bought the Street Pilot III from Circuit City and it comes with the map of the entire US, Canada, and the Virgin Islands. I can buy Europe and Asia map if I want. Paid $999 but found an ad on the internet and return to Circuit City for refund plus extra 10% matching. Total cost was a little over $700. Now I can use the unit in any of my vehicle.
Come to find out it is the same unit they sold back in '98 for the ML. Or at least very close to it. I saw that in the Road Side Assistance ML when the guy came to my house to jump my car couple weeks ago. I asked him if it is a Street Pilot III on his dash and he told me it is something similar except it comes with the MB logo. The only thing I wish I have is the voice command that comes with the higher end units. Guess Garmen will have that feature in a year or so on the Street Pilot III.
04 Infiniti G35.5 BS