alternator problem on 2000 jetta tdi

in cold weather only, the battery light flashes when driving, not idling. ive put 2 new alternators on the car, had it to a VW dealership and discussed the problem with BOSCH to no avail. chscked all grouds and can not find anything wrong. when light flashes the alternator is not charging. been stuck a couple times already. anyone have this problem.. please help!!