LED brake lights - detecting voltage drop?
It's probably not that simple.  How did you measure the voltage level, with a multimeter I expect?  It is probably a pulse stream with the information actually encoded in the length of the duration of the positive pulses.  Probably the pulse varies from 1 to 2 ms.  This looks like a voltage signal only because the meter averaged it over time.

If all the above is correct, then to use this signal you have to decode the pulse width.  There are various ways to do that with most microcontrollers.  The micro measures the pulse width, decides whether that indicates reverse or not, and turns on a output accordingly.

A simpler way to do this is to let your existing decoder decode the signal and see what it decides.  Ultimately it's going to control the motor.  You could have a transistor turn on whenever a negative voltage is applied to the motor.  Probably the controller is pulsing the motor between 0 and full supply voltage when going forwards and 0 and negative supply voltage when going backwards.  The fraction of time it spends on the supply voltage versus zero is how hard it drives the motor.  By detecting negative voltage accross the motor, you should be able to tell when the controller is trying to back up.