Tachometer wiring

I am adding an aftermarket tach to my car that did not have one from the factory. I am not knowlegable about wiring. The wire to the coil and the ground wire I can handle. Looks like I need a 12v switched ignition source and another wire to light switch for the internal tach illumination (not shift light). How and where do I do this without hacking into my harness? Thanks!