There seems to be a lot of confusion regarding where/how to find the so called tach wire. Here are a few comments:
The name of the game here is to get a signal that typically is an alternating frequency that is varying at a frequency that is representative of how many RPMs the engine is turning so that the RS can determine if the engine is running or not. In my opinion, this is the only way to go especially in cold weather for more reliable starts
The "old" style ignition system is pretty straight forward. A cam shaft inside the distributor opens an closes a set of contacts(points) which when closed, allow current to flow in the coil primary. The frequency of these "pulses of current" are directly proportional to how fast (RPMs) the engine is running. So merely running a wire to the "hot side" of the coil primary ( the other connection on the coil is typically tied to ground) and we have the "tach signal" for the RS system.Note that we need to run a wire through the firewall to tap this signal on the ignition coil.
The "new" style electronic ignition systems (aka "waste spark" systems )is whole different world. This system is computer controlled and basically uses three sensors ( 7 x Crankshaft position sensor, 24 x Crankshaft sensor and camshaft position sensor) to calculate engine rpm, when to fire the injector solenoids and how much to advance/retard spark among other things. So there really is no "tach wire" per se.
The three sensors typically are two "hall effect" and one "reluctance pickup" (the 7 X crank sensor which is the HEART of the system without which the engine will NOT start). So it probably is not a good idea to try to tap one of these signals to use as the tach signal. Probably the best way to "create" a tach wire is to pick off one of the fuel injector solenoid signals ( any one will do)right at the Power train control module ( aka "the computer"). This way there's no need to go through the firewall. However, you must be absolutely sure of the correct wire for as we know, a screwed up 'computer" replacement is huge bucks. Undoubtably, the best way to get it right is to have the actual service manual for the vehicle which will give the computer connector signals and pin outs. Basically, each injector solenoid has 2 wires. One wire is "common" ( the same color at each solenoid) and is tied to +12V through usually a 20 amp fuse labeled INJ/COIL in the fuse panel. Each individual other wire on each solenoid goes through the harness and back to the computer which switches (pulses) that wire to ground to activate the injector at a frequency proportional to engine RPM . So all you need to do is tie on to one of these wires ( identified by the service manual) and bingo, there's your "tach signal".
One other key point about computer controlled systems. If you disconnect power to the computer, it "forgets" stored emissions control data and the car ( in most Startes) will NOT pass inspection because the emissions tests will read "incomplete" until the car has been driven about a hundred miles under various speeds , engine start/stop, load conditions etc and the computer has a chance to compile a history of "good" ECM data. So to keep your customer happy, take a look at the inspection sticker. If the car is due for inspection ask your cusdtomer to get it inspected before you begin work. This prevents the situation where the guy comes back screaming at you that his car won't pass inspection since you worked on it.
Hope this long (too long) post helps someone.
" The man who knows "how" will always have a job.....the man who knows "why" will always be his boss"