Switch question for rear defroster

You can estimate the resistance pretty easily. For instance, if it is a 12 volt supply and you measure the run current with a meter and it is 2 amps, the 12 v divided by 2 amps, that means the motor has an effective resistance of about 6 ohms. For starters, if we add 2 ohms (about 30%) that reduces the current to about 12v divided by 8 ohms which is about 1.5 amps. So you have reduced the current by 25%. The motor speed isn’t exactly reduced by 25%, but it is a good ballpark way to start.