I think it's probably 12 volts, man.
While the '48 originally would have been a 6 volt system, they probably went to 12V when they put the newer engine in there.. the real way to tell for sure i'd think would be to look at the metal tag on the generator.. it outta say 6 or 12 volts.
Pulling the cables off and having the voltage drop slowly suggests to me that, assuming you have a new or for sure good battery, the generator isn't putting out enough juice.. the fact that the gauge is leaning on the "D" or "Discharge" side seems to confirm that.
As far as how to turn the wipers off, in the '50's theres this silver "chicken head" sort of knob right on the dash by the windsheild.. turning that off outta turn them off and on.
Big Bolts kick [censored], man.. and a Big Bolt dump.. well that kicks [censored] x 2!
