Overdrive relay

For the '50 and later cars, Fifth Ave's headlight relay will do the job for quite a bit less money, unless you want something that looks similar to the original. As for a resistor for the solenoid, that's a tough one unless you have a '49 and earlier solenoid. Problem is the later solenoid has the high-current "pull-in" coil and the lower-current "hold-in" coil connected to the same terminal. A set of contacts in the solenoid disconnects the pull-in coil once it's done its job. So if you size a resistor to drop 6v when the pull-in coil is active, it'll have hardly any effect on the hold-in coil, so the latter will still be receiving most of the 12v. If you size the resistor to drop 6v when only the hold-in coil is operating, then when the pull-in coil is trying to do its job, the resistor will drop way too much voltage, and the pull-in won't (pull in!). I've worked this problem by using the '48-'49 solenoid, which has separate connections for the pull-in and hold-in coils. One can also search around for a solenoid from on a 12v OD equipped car. I've got a Studebaker 12v solenoid on my Hornet.