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/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.
) 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