Damn man , I cant find any info on istalling a aftermarket tach on a coil like our Isuzu's use.
If it was the old cylinder style with the + side and a - side with the coil wire in the middle , i would have been done long ago.
Sucks , because its all mounted up and wired up , minus my engine rev wire(coil wire).
The plug that goes into the coil, it has two wires I think, the only real difference between the old style and ours, is ours is a snazzy plug with a water resistant gasket. I'm out of country, so I don't have mine to reference. One of those wires has to be the one you're looking for. I'll see if the wife can check out my electrical manual and tell the one you want. I can't make any promises she'll know what she's looking at but it might help.
Ok I talked to the wife and she looked up some info.
Theres two wires in that snazzy plug and the one going to the distributor. Heres what she was able to find.
"black wire to coil" probably the spark plug looking wire.
"blue to primary output control" Not sure really.
"Black w/ yellow stripe" eventually goes to the dash fuse box after several connectors, then to the radio noise suppressor then ground.
I'm not sure which one you are looking for, but thats all we have under there. Now this is all for a 90, so yours might be different.
The one I would recommend would be the black with yellow stripe, cause it goes to ground eventually, should be the negative side.