It seems that the direct ground lead solved the player dropping out. At least that's what 10 minutes of testing shows so far.
I checked the charging system voltage with the motor at various rpm- all nice and happy- so I doubt I've got any alternator or battery problems.

But now the problem is that my ground-loop whine is louder than ever.
Before, I could reduce the whine by pushing on the face of the player (on the bolt heads - don't hate me). Now there is no 'wiggle solution'. Can't say why pushing used to help.
I tried holding my new negative terminal lead onto the sled and empeg housing- it did alleviate the whine by about 80%. I'll have to find a screw to fit that threaded hole at the back of the sled.

If I can't find a screw or that doesn't solve the whine I'll have to re-install my old head unit for entertainment on the drive. Out would come the empeg for now.
Feel my pain.
Grrr.