[patrickt]

Just run a long wire, a lamp cord cut up and spliced together will do nicely, but run it to the negative terminal of the battery. There are short cuts, but they often mislead you. If you really do need a supplementary ground at the alternator, then your VOM might read a half volt or even one volt. One volt would be a lot. But by all means drive your car. At this point you are only optimizing; you're not fixing anything that's really broken. FWIW, when I run a VOM from the alternator case to the negative terminal of my battery I read .05 volts.