Just a bit of information - GPS antennas need to see 'sky' - usually thru the windshield. The placement possible with some aftermarket mounts could reduce the sky view. An external antenna such as: http://shop.garmin.com/accessory.jsp?sku=010-10052-04

mounted on the roof is a solution. The improved reception yields better location accuracy.
dave and Xtoy - in a smaller circle

------------------