Since both gauges are not working I would verify that you have voltage at the regulator. The 6v input is the terminal toward center and with the meter leads between that terminal and the cluster as ground there should be 6.3v or battery voltage coming from a circuit breaker next to the regulator when the key is on.
The terminal toward the end is to the instruments and is a pulsing output which because of the frequency and length of pulses will average 5v to the gauges if the regulator is working. The frequency and duration of the pulses depend on the load so is not a fixed number. Without a very fast responding meter or an oscilloscope you will probably not be able to get any kind of decent reading at that terminal although being 6v you might because the pulses will be fairly wide. It is hard to do any internal measurements because of the construction.
If the meter display tries to read and you can get some kind of voltage reading on that end terminal that is a good sign. If you see a constant 6v out that is an indication the heater element might be burned open and regulator is not functioning. If you see no voltage at all then it could be the meter or possibly the contact points are burned or otherwise damaged. In either case swapping to a known good regulator is the procedure.
Here are a couple of photos showing what the reg looks like inside and what the output looks like at 12.6v input. On 6v the height of the pulses would be just over 1 division on the scope and the pulses should be much wider.
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