Modern Vespa

On my GT200, my engine temp gauge hasn't worked for over a year and I'll be taking the headset off this week to replace the broken speedo so wondered if I can fix both at the same time. The thermistor appears to work but the dial only flickers intermittently when I touch it with the wire. I'm guessing a broken wire somewhere and thought about replacing the wire completely.

Any idea where the wire connects on the control panel?
How to route the wire from the control panel to the engine compartment?
What type of wire to use?

Not that common. More likely corroded or loose connection or bad sensor or gauge. Manual if you don't have it yet.

http://www.wotmeworry.org.uk/manuals/Vespa/GT200/

I tried swapping out the thermistor but the new one didn't fix the issue. I then figured it must be the wire (probably in the engine compartment) so I cut this wire out and retried. No fix. The dial does occasionally move as I try shorting the wire to the engine but it's doesn't always happen. My current guess is that it's a bad wire or loose connection in the headset. I could test this if I knew which wire I was looking at.