[T3] Mystery No Start

[J. Jonik]

'71 Sqbk FI Std.
30 mile trip in rain, wipers and lights on.  Turned car off to unload things at home.  After 2 minutes, car wouldn't start...like no battery...only noise the FI relay thing. But headlights were fine.   Pushed car into slight down-hill parking spot.   Snugged up all relevant wires, moved and re-tightened pos and neg on battery (and neg to ground), and gravity test had battery being ok.   Still dead. Luck had it that a neighbor, very soon, said he had a battery charger....or "booster"...the kind, about size of a toaster, that you just need to clamp to the battery to jump it.   Miracle device...never used one before. (His thing also could do air for tires.)

Car started right up.   Started again (without help) a few hours later.   Started again the next day and all six days since then.   Auto Store ran battery test today. Battery good.  But, when will that oddity happen again?  A once-in-lifetime fluke?

So....what happened?  Can a fault in wiper motor, with (or without) lights on, do some dirty trick?  Is there a way to test if wiper or lights are messed up somehow?
A clue to something?  Just before that 30 mile return trip, for a brief moment, the wipers got Very Slow, almost stuck...then returned to normal.  But car started ok after that.
Where's a "complete idiot" manual for 45 year old VWs?
   
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