Measure the voltage at pin 6 on the regulator connector. If it's greater than the voltage at the battery, trace the black wire from that connector to the battery.
If it is the same as the battery voltage, check again on the regulator side of the connector. If it is higher there, check the connector pins for damage and corrosion.

Let's hope there's intelligent life somewhere in space 'cause it's buggar all down here. -- Monte Python