tach install??

I have an 87 pioneer with no tach, I want a tach. Should be an easy problem to fix right, just install a tach. I've seen some wiring diagrams for coils like on mine and there's 4 wires and a blank for the tach feed wire to hook. Well mine has 4 wires and no blank.

Anyone ever installed a tach in one of these? I've searched the internet and havnt found any fixing my problem.

I had an old inductive pickup from another tach I never used, figures cut the wire and splice it in. Wrong, it was a shielded wire and still won't work.