Originally Posted by Oilyrascal
......I can tell you for sure the resist depends on the current flow. On a blower motor it would probably work, but on the wiper motor, with a dry windshield, the motor has a lot of drag on it, so it draws a lot of current through the resistor, so the voltage has a large drop, and the motor barely moves the wipers. When the windshield is wet, there is less drag, so there is less current flowing to the motor, so there is less voltage drop, so your wipers are whipping back and forth too fast. It's not a good thing.
This is correct. The amount of voltage dropped by a dropping resistor varies in proportion to the amount of current passing through it. This applies to all resistors, whether they are aluminum, ceramic, or anything else. Voltage dropping resistors are only suitable for heater blower motors, and lights.
What I recommend is an electronic voltage reducer. It's a transistor based device that senses and reacts to varying current loads to keep it's output voltage a steady 6 volts.
I have had extremely good luck with the Ron Francis VR-4
electronic voltage reducer. it's rated for up to 15 amps continuous.
I have installed at least ten of these over the years, and only one has failed - and that one only failed because it got overloaded.
Hope this helps....