new member , not sure about posting so any advice welcome . I have a reliable 1.9 tdi 2006 but it has developed an electrical fault where the  tail light and indicators fail when the head lights are turned on . checked all fuses and cleaned and checked bulbs , during my investigation I found my ground voltage is 12+ volts and can deliver about 4 amps (backwards???) when engine (alternator) is off .I am now going to isolate the various circuits to try and isolate cause . voltage is present when key is removed so a suspect is the starter motor but I am presently flummoxed . any advice or sharing of similar experience would be appreciated . I should also note car seems ok when running but lights are not properly operational when marker /headlights are turned on , and this apparent short circuit is lurking in the shadows. 

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Edited 8 September, 20205 yr by Wino

willdo wino thanks

hello again to all contributers . Just a final post to let members know I solved my problems , it was caused by an insulation piercing connector on the earth line to the towbar which had broken  the wire , thank you to all who took the time to offer advice.