If we're talking about the factory GPS, it spent about two months telling me I was in the middle of the Columbia River off Hood River, so welcome to the club.. . . Now I have to figure out how the GPS will discover that I am about 4 miles from where it thinks I am... Any suggestions...???...
The problem is that the signal getting to the GPS receiver isn't strong enough. It was marginal when the cab overhang was half what it is now, so I'm pretty sure that they'll be too much in the way of getting a good signal now.
It's been a long time, so I'm not recalling all of my investigations, but I believe that you'll find a factory GPS receiver "puck" attached to the radio under the dash cover on top of the head end. I recall that I pulled it out a bit so it rode on top of the dash and that made the situation somewhat better. But it was never really reliable, and I eventually got a separate GPS which I RAM-mounted to the dash bar. That worked for me, though to be honest, with the new roof, even that might not do the trick. You might borrow a GPS unit from someone to test it. If that doesn't work, you'll need to see if you can find a unit with an input for a remotely located antenna.
BTW, and FWIW, there's a rudimentary GPS built into the Yaesu dual-band radio, and that has a remote GPS receiver; the puck is in a small white box. That setup will give your coordinates fine, but no hint of navigation.
Good luck with it.