Need troubleshooting assistance

As title said, in need of ideas. Lots of smarter than me folks here. Car is 1970 Opel GT, Stock. Back story: haven't had dash lights in a while so a finally got around and pulled the dash. Got the lights fixed, seems the last light in the circuit is keyed and wasn't aligned right. Now the lights work but lost the combination fuel level/engine temp. Have power to the gauge but still don't work. Disconnected the gauge and applied a small amount of voltage from an external battery (about 3 volts) and the gauge pegs. Checked the sending unit at the fuel tank. It reads about what I think it should with the amount of gas in the tank. Every time I try to take a direct voltage reading off either gauge it pops the car fuse. I will add that before all this the gauge would stick once in a while and a tap would unstick it. Gauge is original to the car and VDO. Dual fuel quantity and engine coolant temp. Simple operation. Power to the + on the gauge and the ground goes to the fuel tank sending unit with the float variable resistor handling the reading. The temp part shares the power wire and goes to the temp bulb next to the thermostat on the engine. Simple basic system but it is driving me batty. Guessing now that the gauge could be bad. Any ideas for conformation?

My wild ass suggestion: pull that last bulb out that was installed incorrectly. Does the guage work normally after that? Is there corrosion on any of the bulb connectors or ground wires, or the power wire that connects the temp gauge and the warning light?

Sound like a short to ground in the gauge circuit. Broken insulation, bulb weirdness, power to the wrong spot?

I guess I should have mentioned that last bulb in the circuit that is keyed is in a rocker switch not associated with the gauge. No warning light. Cleaned the corrosion off. Thought short to ground myself but have continuity and power in all the wires at the terminal ends. Power is seems to be flowing in the right direction and where it's supposed to be. Haven't figured out why when I touch the multimeter probes to the terminals (the right direction, I double checked) it pops the fuse. Digital Multimeter set to DCV 20 volt range. Fuses for this car are hard to come by and I'm running out. Uses the old European GCI fuses. NAPA is the only place that carries them and they are across town. Not expensive but pain.

Keep 'em coming though. Just like a checklist, if I hadn't tried it multiple times already I'll try it again. Want to get the car back on the road, it's driving season.

Just noodling through this. Different circuit putting power back to ground? Have you checked through the whole fuse box for other weirdness?

Any chance you swapped the wires around? I know that causes all kinds of grief for the oil pressure gauge and idiot light on the GT.

On my Kadetts the gauges need to be grounded to work right. In their case it's originally the speedo cable and one particular light designed to ground them but these days I just run a dedicated wire as a matter of course. Iirc the GT has a bunch of grounds that tie together between the two main gauges; I'd start there.

Seems like the fuse in the multimeter would blow first if you were shorting something when checking the voltage in the circuit. Wierd.

How is the ground at the temp sensor, assuming there is one?

Tangent: do you have any dual filament bulbs on the light circuit? High/low beams or parking/turn signal bulbs all working correctly? My thought here being that if the dash illumination lights share a common ground with the temp/fuel gauge, and there is any short in the light circuit, that the amperage from that circuit is what's blowing the fuse when you are measuring voltage on the gauge.

A multimeter on a dc volts scale should have such a high input impedance that it never should result in a fuse bliwing. Are the leads connected to the meter correctly? Is one plugged into an ammeter connection?

Just to close out this thread. When I started this fiasco, I did think it was a ground issue too but couldn't find the bad ground. Went through the whole system multiple times and just couldn't find anything wrong. Followed the advice given and just couldn't find anything wrong. And yes T.J. the multimeter was hooked up right. It's a cheap HF digital multimeter with only one connection for each lead. Went out this morning and triple checked everything again. Been thinking about this for a couple days and thought there was a ground that I missed but that ground didn't affect anything. Found a loose connection at the fuse box, I know I checked those connections numerous times but somehow missed it. Must have not been seated all the way and didn't get a good connection and finally just dropped away where I can see it. Where's the bang-head emoticon. Fuel gauge now works. Even after I reinstalled the instrument panel, steering column and tail lights. Even the instrument lights are still working and that's what started this whole fiasco. Now to take the car out for some exercise and celebrate.