The Navigation unit GPS antenna on the H2 is mounted under the top of the dash just to the right of the I/P cluster (very similar to the nav antenna in the H3). On the roof you probably have a black wart with a tiny antenna. The wart itself is the Onstar GPS antenna and XM antenna (two different antennas in the same black wart), the tiny antenna sticking out of the wart is the Onstar Verizon communications antenna. (If you managed to find a H2 with no Onstar, all you will have is the black wart with no tiny antenna.) (Wart is what we called it, but it is really known as a muilti-function antenna array.)
The Gobi rack should cause no interference on the GPS, like it will with XM. XM, basically works off one satellite, while GPS works off 26 (maybe 27 or more now), and almost at all times, a nav system should see about 8-12 sats.
I installed my Pioneer nav antenna in almost the same spot that the factory antennas would be on my H3, and if I monitor the number of sats picked up, unless I am in a deep canyon, I usually see at least 8 and all you need for a good fix is four. Also, the software used by nav systems are programmed to lock onto the road, not onto the exact spot. So if the actual location is down to approx. 30 feet, if there is a major road, the nav system should show you on that road (it's in the software). This can sometimes cause problems if you have two roads parallel to each other and very close (service drive on an interstate). But usually, the GPS should get you down to around 8-10 feet easily.
