What do you mean that you have +12 to comm, and -12 to N.O. The common and the NO get connected when the coil is energized. Why would you want to connect the - and + 12 volts. How the thing works at all amazes me. Something just doesn't seem right.
Please revise/confirm your wiring description.
If it is by chance connected correctly, check to see that the accessory wire is actually dropping to zero volts when the ignition is turned off. Some cars can keep an accessory wire "hot" for a period of time after the car is shut off.