aftermarket tacho: unknown brand: what are the wiring connections?

[wagonist]

I got an aftermarket tacho in a wreck I bought.

It has no brand name unfortunately, but is switchable between 4, 6, & 8 cylinders and has an adjustable shift light.

There are 4 wires coming out:
White, red, green, black.

From pulling the back off, it appears the black & green wires are across the illumination light, so I'd assume:
black = earth
green = illumination

Now I can only guess that the red is ignition 12V & white is the tacho signal as this is the normal convention.
If I hook it up this way, what damage can be done? (usually the tacho signal is an negative)

[dumbass]

Red is ign
Black is ground
White is light
Green is rpm in

[wagonist]

Except I've already traced the green wire to the illumination lights by removing the back and using a multimeter.

[dumbass]

Never seen white wire as signal on a tacho in 20 years
But anyway just put 12 volts down the green see what happens
Not my tacho hahahah

[wagonist]

Fair enough. It's my first one.
I'm not aiming to keep it it's just I'm doing an engine conversion & the donor car has a full-microprocessor controlled instrument cluster, but the recipient car doesn't. The car this tacho came from is the same as the recipient car, but I've wired in a factory tacho cluster instead (except it's small).
I think I've found the tacho wire into the cluster on the donor car & just wanted to wire this one up temporarily to check.