At the washer motor, the black wire goes straight to chassis ground. The blue/red (or other non-black) wire goes through a connector at the left A pillar to the wiper switch which provides the 12 volts. When you got 12 volts on the connector with the test light, it's not clear if you were doing this with the connector on or off the motor. If you're saying that, with the connector off the motor you were testing between some chassis ground point and the connector pin for the black wire then that isn't correct -it would mean the black wire is broken somewhere and shorting to a live wire (highly unlikely). More likely you had the connector on the motor and were testing between some chassis ground point and probing into the back of the connector with the tip of the test light. If you saw 12 volts on both pins then that simply means the black wire is not connected to chassis ground. Unless the black wire is broken somewhere, just clean up the ground connection where the black wire goes to the chassis on the left wing and all should be well.
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Dave -own 940's, prev 740/240/140/120 & quasi-expert only on a good day