You don't want to do voltage drops between the pins. You would want to do voltage drops on the paths themselves. I'm thinking that you're putting one lead on the pin and then putting the other lead on a ground... that's not a voltage drop (well, technically it is). A voltage drop on a path is placing one lead of your DMM on one end of a wire, and the other lead of the DMM on the other wire and taking a voltage reading with the circuit LIVE (ie: nothing is unplugged and the circuit has current flowing through it).

The green/white wire on the rheostat is hot when the park lights are on. The black/yellow wire will vary it's voltage based on the rheostat knob position from 0V - 12V. The solid black wire is a ground aka: NO voltage on that wire.

Now, with that being said, do your dash lights illuminate at all? Does a fuse keep blowing? If you wiggle the dimmer switch will the lights on the dash flicker?