I had exactly this issue and it turned out to be the GPS receiver in the roof fin (mines a 2006 4.4 Vogue SE). The roof fin plastic had degraded, had gone brittle and had split, thus allowing the weather to get it.
I was lucky in that I had a stand-alone GPS puck from a previous car GPS. I plugged this directly into the satnav unit and the navigation started working normally (red line disappeared). Then I had to buy and replace the roof fin. It's a simple(ish) job. I think there's a youtube video on how to do it.
Don't know if this is your issue, but it's something to check if all other solutions don't solve it.