I have heard of a few people mounting their gps ant ontop of the actual head unit chassis installed in the dash. I have always mounted the thing outside. Any one have experience with putting it inside the dash and had good reception?

Alternatively, could one mount the gps ant in the corner of the window upside down? Does it matter that the top be toward the sky?

It should be mounted righ-side up. There is metal (magnet) on the bottom - that can not be good for reception. Speaking of magnet - it should be attached via the magnet to a metal plane. This increases the reception sensitivity.

Mine is mounted in the front compartment under the plastic windshield wiper cowling. At first I just magnetically stuck it on a metal ledge on top of the firewall.Later I stuck it on top of the wiper motor - it improved the signal.

I'm sure every vehicle is different. Ideally, it should be positioned to allow reception from multiple satellites in all directions. The sat signal is very low power and doesn't go through metal vey well.

is it bad to mount on-top of the HU? Will that magnet interfere with the electronics in the HU?

I don't think the magnet will interfere with the HU. But if the HU is mounted under a metal (vinyl/leather/fabric covered) dash, it might not get good signal reception. I guess you could try it, and if it works - good. But in marginal sitiations (heavy overcast, storms, high buildings, etc) you could still have problems. The best location is where the receiver has direct line-of-sight to at least 3 satellites. The more -the better. The GPS will use them all to get the most accurate fix possible by averaging-out reflective and/or environmental induced errors.

is it bad to mount on-top of the HU? Will that magnet interfere with the electronics in the HU?