Ah, I'm sorry, my mistake entirely. The black/black cable is the EL driver, the red/black is actually the shiftlight.
Both of these have a male plug(a socket with pins inside, in this case) going to the tach. The shiftlight and EL driver have a female plug accordingly.
I'm not sure what wires are coming out of the back of your tach, so I'll list the ones on mine:
Red: +12V
Black: Ground
Green: Signal
Red/Black w/connector: Shiftlight
Black/Black w/connector: EL backlight
The black/black cable is also noticeably thinner compared to the rest of the wires. This is where the EL driver plugs in to supply high voltage to the backlight.
The EL(
Electroluminescent) backlight is a sheet of material, not a single bulb. To make it light up, the driver converts the 12V DC you provide into 333V AC(Measured with a multimeter, backlight not connected). This should be harmless as it's only low current, though I wouldn't try plugging it into the shiftlight output as the electronics might not agree with the harmless part
My EL driver looks like this:
My phone refused to focus in this light
Note that I soldered on the switch, it didn't come with the unit. This post actually encouraged me to finally solder on the other to the switch so I could measure the EL driver voltage, as I didn't have wire when I soldered the switch itself. Lazy as I am, I hadn't bothered yet