My 91 Toyota PU occasionally lights up the charge and brake indicator lights while driving. I have verified that there is no alternator output when this happens but have not determined whether the alternator itself is inop, the regulator portion is on the fritz or there is an intermittent connection in the field circuit. Both times I have taken it to a shop the problem refuses to show.
The problem might be with the brushes in the alternator. Check and see if there’s a removable cover on the back of the alternator. If there is, that’s where the brushes are located. You can aquire a new set of brushes from the dealer for about $25.00.