dmm reading 12v

Sounds like it is a reverse polarity switch/device.

Try using a ground reference that isn't a part of the switch (like the chassis of the vehicle).  When referenced to the chassis you should not read -12vdc.  The only time I've seen actual - voltgages in a car is when looking at certain wheel speed sensors and certain crank angle sensors (sensors that measure rotational speed by creating a sine wave).  If this is your case (which it doesn't sound like it because you wouldn't actually read -12vdc) you wouldn't want to use either signals to drive a relay.

And to clearify your question:

Your volt meter reads voltage potentional between the ground leand and the positive lead.  By displaying a -12 it is telling you that the voltage potential on the black lead is 12vdc greater then the potential on the red lead.  This is why it is important to use a 'true' ground, so your voltage reference is correct.  If you choose a voltage reference that is higher then 'true' ground you'll get all sorts of weird readings.  With more and more 5vdc circuits creeping in to cars this is even more important, as you can get some really strange readings if you don't have a true ground and you're metering wires.