One of the things I had to do was reverse the direction of this motor because it spins the wrong way for my car. The most immediate question is: "Why don't you just reverse the polarity to the motor?" The answer is because a series wound motor will spin the same direction no matter what polarity you give it.
This is because when you reverse the polarity to the motor, it changes the direction of magnetism in the armature. But because it is series wound, it also reverses the direction of magnetism in the field at the same time. The result is a motor that still spins in the same direction. A series wound brushed motor runs fine on AC (alternating current) as well. Many small household appliances and power tools use them. (In case you care).
This means that to change the direction of this motor, we have to change the polarity of either the armature or field, and not the other. What I did to accomplish this was change the wiring inside the motor to reverse the polarity of the armature only.
I had to un-solder and re-solder the cable on one power post:
And bend one field power connector to hook it up to a different brush:
After I got it all back together, I hooked up 12 volts to it to test it out: