Hallo Corbin,

I would proceed with the following circuit:

Your supply to motor, which can be 12V or 6V is named 'IN'. Output to your motor giving halfed voltage is named 'OUT'. Then build the following circuit:

From IN to ground a voltage divider: Connect R from IN to point A. From point A to ground a series combination of diode and resistor R. Both resistors are identically.
Now connect a transistor with collector to IN, basis to A and emitter to OUT.
Additionally connect two protection diodes: From OUT to IN and from OUT to ground. (Anode of the first diode to OUT and cathode of the second diode to OUT.)

Diode in the input voltage divider compensates for voltage drop across basis emitter junction.

Bye,
Kai