was charging very low, down in the red section.

This was intermitent.

Replaced voltage regulator, but didn't help much.

then installed a used alternator, and put the new brushes on, and it seems much better. The old alternator had some groves worn where the brushes touch.

but on cold foggy nights, still having problems.

Charges fine when "bulb out" warning light is on, but starts to charge low when that light goes out.

This problem mostly occurs when car is cold, and its foggy out.

All the warning lights will come on for a second or so, especially when driving slow, or turning.

Keeping the revs up seems to help.

I can't find any loose conections, at least near the alternator.

I do see a bit of corresion appearing at the positive starter cable.