Another setup
We use a Garmin Etrex Vista and place it on the dash. We hook it to the laptop and use DeLorme's Street Atlas USA on the computer.
One caveat... . The Garmins I have seen take ONE cord, so you need to get the combo cord that has both a cigarette lighter plug to power the GPS and a connection to the computer for data. They make a data cord, a power cord, and the combo... Use the data cord, and your batteries will run down in the GPS. . Use the power cord, and you can't hook it to the computer...
The Etrex will not take an external antenna, but we have never found one necessary. .
DUH ... Now that I think about it, I actually have the Earthmate on the dash, and use my Etrex mostly handheld for geocaching. The Earthmate drawing its power from the laptop is nice, but if you ONLY have it, you can't use it independent of the laptop... .
Going today to likely pick up a lap top stand for the truck...