Earth location?

Hi All,

1973 3500S:

I appear to have missing earths on nearside horn and inner headlight. I’ve disconnected the wires at the horn and headlight bullet connector respectively, and there is no continuity between the wire terminals and a known good earth. The wires disappear into the nearside wiring loom and into the dash area, I assume that they are bolted to the body somewhere, but that is my problem - where? There is one small earth bolt above the passenger glovebox but I don’t think that is it.

Obviously I can make an earth cable and fit it to a convenient bolt (coil bolt maybe), but would like to try to fix it properly. The repair manual is no use, and I haven’t found anything on search. The puzzling thing is that all other earths in the front nearside corner are fine.
I guess someone who has fitted a new loom would know….