Running a bit hot.

you're looking at the main earth back to the batytery, the mk1 position on the gearbox bracket isnt the besat place, later mk2 and mk3 use a stud on one of the main engine -> gearbox mounting bolts so i would add one of these and run an earth to it from the battery

Just in case it isn't an electrical issue….. You still got the old thermostat? I fitted a new one to my jetta when I fitted a new water pump. Both parts supplied by VW. I then had overheating issues. I took out the new stat and sat in in a pan of water alongside the old one. I brought the water to the boil and the old one opened faster and wider than the new one so I stuck it back in and the issue seems to have been resolved.

DubPartz said

Just in case it isn't an electrical issue….. You still got the old thermostat? I fitted a new one to my jetta when I fitted a new water pump. Both parts supplied by VW. I then had overheating issues. I took out the new stat and sat in in a pan of water alongside the old one. I brought the water to the boil and the old one opened faster and wider than the new one so I stuck it back in and the issue seems to have been resolved.

I had the same issue with the old thermostat mate, same place on the gauge when warmed up.

Not had chance to check the earth out yet as the Jetta is leaking rain water in through a factory hole in the chassis.