door trigger problems

I am having a very hard time getting the door triggers to work in a 99 chevy silverado.  I know that the tan wire is for the driver door and the blue/white wire is for the passenger.  The problem is with the tan wire.  It shows ground all the time, whether the door is open or not.  However the domelight cuts on when you open the door and stays on until you shut it, just like it should.  i tried to tap into the wire on the domelight that turns to ground when it's on, but that doesn't seem to work either.  Here's where i'm having trouble.  Ok, i'm at the dome light switch in the dash.  There are two lights that go to the dome light, an orange and a white.  Both show +6v when connected to vehicle ground.  However, when the red probe of the dmm is connected to the orange wire and the black probe to the white, it shows +6v, if you reverse the probes it shows -6v.  They show this when the light is on and off.  Now this doesn't make sense to me.  Can anybody help?  What makes the light turn on and off and how can i interface with it?

I found a way to do it.  It didn't make much sense though.  I used the domelight wire at the courtesy lamp inside the driver door.  At the domelight switch it read 6 volts but at the light it read 12.  I just used the negative side and ran it to the door trigger.  Now it performs just as it should.  Thanks for your help.