is this for a vehicle? if so does it have an old fashioned contact breaker/ coil type ignition?
I got a pair of them, along with a bunch of other gauges (fuel, pressure, amps, water temp) from a boat console I salvaged, twin engines, I don't see any switches for glow plugs and I do see a switch for a blower which I guess tells me it's probably gasoline rather than diesel as I had first thought.
there's a way to simulate this in hardware otherwise ..... but are the sense wires you're connecting to totally isolated from all other connections? - to confirm my guess of an internal pulse transformer?
The wires are all cut, however there are two yellows coming of each ignition switch which could have been the two yellows that went to each RPM gauge.
The sensor leads on the gauges are open loop to the lamp power and ground on the gauge.