I will probably mount an earthed spring in the back of the sled which will earth the AC RCAs to cure my ground loop. I say probably because I've not decided whether or not to try an isolator on the RCAs instead. Currently stuck between the idea of modifying the sled [guaranteed cure] or buying an isolator [result presently unknown].

On the subject of Ground Isolators: What exactly are they, small transformers?
I will be going to a scrap yard in a few days, might I possibly find them factory-fitted to certain cars, if so what make (& model)?