The factory ran the wire through the firewall connector as part of the engine wiring harness. My car never had a tach so no connectors.

I've run various extra wires through the rubber firewall grommet that the vacuum gauge hose and temp sensor wire run through, so that's what I'll do for this wire (only one wire needs to enter the engine compartment, the one that goes to the coil). It's a little round grommet near the center of the firewall. It had holes for two tubes/wires in it - I just cut the middle out of it to fit more wires through and it looks factory. Might be a different setup on your car.

I'm also psyched that Greg makes a replacement board - I have no idea if my sender still works.