with any aftermarket tacho, there are usually 4 wires
12v Positive. you hook this up to an ignition-switched power source, such as the part of the stereo that comes on when you switch the key on
12v Negative. you earth this to the other part of the above power source, or just to any bare metal place on the firewall or dash
12v Lights. this goes to a psotive power source that comes on when you turn the headlights on. the little dash light is a good place to steal this power from
Signal. hook this up to the negative side of the ignition coil. yes, the one in the engine bay. you'll need a big long wire. some tachos say to hook up to the positive of the ignition coil, i've never really found any difference, except that would probably stealing some of your ignition voltage (this is bad) wheras it wouldn't on the negative side
colors vary from manufacturer to manufacturer. i've seen red, black, white, yellow, green, purple and orange tacho wires. that's more than 4 colors (out of 4 possible things it could be) so if you're unsure of what's what, contact whoever makes your tacho (look on google or in the phone book) and ask them, we more than likely won't know