That is a good question, John. If you mean can you make a single speed motor run faster, the answer is not without ruining it and usually ruining it spectacularly. A two speed motor will have 2 distinct coils inside...there will be a low speed coil and a high speed coil. It has to do with different magnetic fields produced when each coil is energized. If you have a single speed motor you want to slow down, it is possible to produce a voltage-reducing circuit (or to simply use a 12 to 6 volt resistance mechanism) which will just give the motor 1/2 of the voltage for which it was designed. Then the motor will run at half speed but it is important to understand you will probably also lose the torque produced by the higher voltage and it is likely your motor will run too hot.
The tricky part is: If you limit the motor to 6 volts, it will run but it will it try to draw the necessary amount of current (please remember voltage and current are different...voltage may be thought of as the size of your arm while current may be thought of as how strong your arm is). That amount of current probably won't be available. In fact, you'll need twice the current at 6 volts...but that's another discussion. If you run the motor using a 6v reducer, run it for a while and keep your hand on it. If it feels hot, then this approach won't work. Most 6 volt motors will run just fine with a voltage reducer, but most auto motors are fan motors, etc which don't need much torque. The wiper motor is more like a starter motor in that it produces a decent bit of torque (which is then geared down further). And when the wiper motor starts, it sucks more amperage than you'd expect...most are fused at 20 amps. In a perfect world, you'd turn your motor into one with two separate coil windings. Hope this helps.
