"I get weird readings from the license plate light ckt. When the key is on I get 12.8 volts between the positive wire and the frame. When the key is off for some reason I have continuity between the two conductors inside the plug, as they they are both ground. Does the key ground the positive side when it's turned off?"
You're measuring the resistance of the lamp itself. That's normal.
10 Duc Streetfighter S (black)
67 Triumph Bonneville TT Special