Re: where does this wire go
Did you change the coil because it was not working at all? If so, all coils have a black ground wire from the coil primary winding to ground. If this wire fell off, it was the reason the coil was not working. The new coil should have a black wire grounded to the mounting bolt. I forget exactly where the main ground wire attaches to the electronics mounting plate but it may attach to the same bolt and a bolt on the bypass cover.
The mounting plate is insulated from the block with rubber vibration mounts and the factory grounded it with a small black wire. I never liked this system of grounding and on my Prestolite systems, I "daisy-chain" a ground from each coil and each CD box to the engine block terminal strip ground.