The metalised windscreen attenuates gps signals big lots. When using pocket pc for nav I stick gps antenna on roof, when using tom tom I put unit up by drivers mirror where clear patch in metalising is.
IIRC data is shunted back and forth using your vhf. It ain't a fast data rate and is prone to lots of re-transmission to make up for lost packets, very sensative to dodgy aeriel leads/plugs/badly mounted aerials. If you only have a quarter wave vhf aerial you might try getting a full wave whip

it doesn't help much with signal strength from car to base but does make a considerable difference in reception of base signal in terms of noise and range. Also put aerial in middle of roof if you can (I'll bet yours is fixed somewhere else just to be awkward

), that also makes a big difference to signal quality and interference rejection

.
http://www.everything4taxis.co.uk/two.htm has a diagram about half way down the page.
ttfn
52 HDI(90) 406 saloon. Sadly no longer owned, bless her she got near 200,000 miles and I had to sell her on, she was still going strong.
Airfix test pilot and part time formula 1 driver for scalextrix