The temperature sensor for the gauge is on the left side of the engine, near the middle of the head, near the #2 and #3 glow plugs. You need a wiring diagram to figure out what wire color feeds the gauge so you can tie in at the cluster. BTW, do not disassemble the big round connector because all the wires will fall out. It's a real PITA to get them all back in the correct sockets, trust me on this. Also, a jump gauge may also be due to a bad ground connection. Do any of the other gauges (fuel) jump around? I fixed my jumpy gauges by running a separate ground wire from the instrument cluster.