The 6 volts I measured is on the output of the course computer. I'm going to compare the voltages at the course computer input vs. the battery bank, to see if the course computer is being fed with a significant voltage drop. If not, I'll try measuring again while the AP is turning the wheel. If no drop I'm not sure what's next.

I'm disappointed that Lewmar hasn't come back to me yet with any questions, answers or anything. Makes me think in terms of replacement and/or adding a redundant AP. (Perhaps a wheel-pilot as backup.) I can't single-hand my boat without a working AP, and even though it's usually two of us it's a real pain to have someone always at the helm.

Edit: The failure occurred while motoring, so the voltage should have been held high by the alternator...