Tach hook up

[Joe Bob]

I have an MSD Capacitive discharge ignition and the tach signal does work the after market tach....I have the negative on the coil hooked to the tach input, proper ground, and switched power.......

I remember on a stock 914/6 a few years back that I had to use an adapter and/or hook into the CD unit someplace instead of the negative on the coil...any ideas?