Gauge Help

A friend bought a 48 Commodore 8.  Its been converted from 6v Positive ground to 12 v negative ground, and rewired with plastic insulated wire.  I have the diagram.  Today's problem is that the two gauges don't work, nor do the idiot lights. The gauges appear to be newly wired.  I understand that Hudson gauges read full when the power is OFF.  But these read full when the power is off and on. I think there is a resistor to them, a little chip like thing wrapped in black plastic, which should take care of the 12v part. but the pos/neg is befuddling me. Any help would be appreciated.