I had a similar problem in my 74 2.0. Are you using the upgraded (slightly smaller, numbered) VDO gauge in the center console?
When I installed mine, I was getting some power to the gauge/sender, but the gauge just didn't work. I bought a second new gauge/sender set - same problem. But the OLD guage worked!
I played with this FOREVER until I started looking at the connector on the new gauge. The wire positions aren't the same on the new gauge. The connector will fit, but wires are in the wrong place. I pulled the old connector apart, put the wires right on the temp gauge - voila!
Hope this helps,
Rusty