Having just examined X5intheLandDownUnder's Mk4, I have to say this behaviour is entirely due to water ingress into the nav. It collects in the bottom tray and starts to electrolytically corrode the electronic components permanently across 12V and ground. Nothing is evident until the terminations on the resistors are completely gone.

Please check for water ingress. Remove the bottom lid of the nav and inspect the condition of the pcb underside.

Address the water leak if oxides are visible.

It's a major job to rectify the problem when it's too far gone.