Check for power with either a multimeter or a test light. The most likely problem is, as suggested, a faulty regulator or a fuse, but could be a wiring connection or even the ignition switch. Note that the output from the regulator is meant to be an intermittent - on/off every few seconds.

John

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JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol