Larry --

The "heavy gray wire" connects directly to the spark coil -- to the same connector attached to the ignition module. One connector has steady 12-volts, the other connector pulses. This is easy to find with a small 12-volt test lamp. This pulsing side connects to the tach.

On an original gas car, the "steady 12-volt" wire should be brown and hook to coil terminal #15. The "pulse" wire should be red-white and hook to coil terminal #1.

On my car old '83, that lead comes out of the main harness along with the other harnesses for the cluster. Others on the BB have said it's there -- but often buried. Since your car was a diesel, it might indeed be missing.

I believe the gray wire is "heavy" because it's shielded -- probably to reduce radio interference. If you find an increase in ignition noise on the radio, you'll probably need to fabricate a 5' shielded lead going from the cluster into the engine compartment (shield grounded) and then to the coil (unshielded OK once inside the engine compartment).