IIRC correctly the blue wire is an exiter wire for the alternator. When the engine comes up to a certain RPM the ligth goes of that wire is what arms the field in the alternator and makes the light go of meaning that the alternator is now producing juice and recharging the battery......

  The tan wire is always a ground in VW cars of the time....

Although this is not the same configuration than the Vanagon, it may help ilustrate what Im saying.
  http://www.vintagewatercooleds.com/tech/Techtips/Timbotechnotes/alternator/alternator.htm#Introduction

  Cheers,
  El T

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