I fixed the same problem with a new antenna.

Else could it be the GPS part in your MK4 whos dead.

After installed my own MK3, it tooked about 25 minutes before it got the right possision.

You can try to push "Set" in the main screen, and just after keep the "Menu"-button in for about 10 sec.

Then another menu will display, and there you can check the GPS status, and see if it have got signals for the satelites, (and s/n-numbers from them)