Which resistor to use

In my vehicle the lock motor wire is 5 volts DC when unlocking and 7 volts dc when locking.  I want to attach a device to the lock motor wire to trigger when it senses voltage.  The problem is it would sense voltage on lock and unlock.  I only want the action to happen on lock.  Do I need a resistor for this applicaiton?  How do I figure out which one is needed?

THANKS

i dont want my device to trigger when it sees the 5 volts.  I want it to trigger on anything over 5 volts say 6.

Basically i want to block the 5 volts

one more thing to add.. maybe I need a relay?

lock motor wire get 5 volts on unlock and 7 volts on lock

unlock motor wire get 5 volts on unlock

heres alittle more info

(+) motor lock wire

      +5 VDC on unlock

      +7 VDC on lock

(-) motor wire

      +5 VDC on unlock

       nothing on lock

is the resistive voltage divider still a good candidate?  maybe a relay would be better.  what I need to happen is if lock trigger one device if unlock trigger another.  im thinking relay due to how they are used with alarms to get 2 outputs out of one. 

If I can just figure out what I need to use, that would be GREAT :)

2003 Audi a4.  i have to use the motor wires for a reason!