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From... nowhere. I guess you'd have to tap into the computer.

Why bother, tho? I know it'd be easier, but just go right thru the firewall. You might be lucky- look for a plug in the firewall by the brake booster. It'll look just like a round disc on the firewall. You can knock it into the passenger area with a hammer and punch (or lonnng, large screwdriver). Then you can lead the tach wire and tach ground into the engine compartment. MAKE SURE you somehow protect the wires as they pass into the engine compartment so they don't rub and short on the sharp edge of the firewall. You could use a large rubber grommet (if you can find one that large), or do what I did, and take a spare 3" length of wire loom, and cover the wires with that. Hell, you could probably also use a spare piece of heater hose or wide vacuum line to protect them. You could also wrap a ton of electrical tape over them.

If you have no plug, you can use the holes in that area as "guides" to make a new hole with a center punch and drill. I'd suggest drilling from the passenger area "out", that way you don't drill into any wires under the dash. There's tons of things in that area that go thru the firewall that you can use as a guide.

You want to patch into the gray connector on your ignition coil, and you want the black/white wire (pin "A" on connector). I would recommend against using a blue wire-tap for this. I used one, and the tap eventually (due to vibration I guess) sliced thru the copper center of the black/white wire. It left me with two strands- and the car was stalling out all over the place.

I'd use a wire stripper to carefully peel back the insulation, then wrap & solder an "extension" wire onto the black/white wire, and cover it with electrical tape. Then run the extension wire over to the tach.