I am putting the engine back together and have a black ground wire hanging from the EFI harness (below the salt and pepper shakers) that I am not sure where it originally bolted up to?
Check the wire color and stripe against the diagram. You may want to use some mineral spirits to clean the wire up so that you can clearly see the main color & stripe. If you don't find it on the diagram, check out the rest of the diagrams on Tmoss & Stang&2birds site.
Diagram courtesy of Tmoss & Stang&2birds
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.
O2 sensor heater ground, very impotant. It bolts to the back of the head, or intake manifold or body where the engine to body ground goes.Just make sure that the metal where you bolt it to is clean and shiny.
I'm not trying to thread jack but I wanted to find out if you MUST ground those both at the same location and why? Head to firewall ground can't be separate?