What Resistor for Dropping 12V to 5V?

Well the values depend on your current draw but a simple voltage divider could be built. PLace two resistors in series with the 2nd resistor value (5/7) of the first resistor value.  Place the resistors between 12v and ground and then you will get 5v at the point between them. This is a very crude way of doing it. It is unregulated so the voltage out will be dependent of voltage in. You will also have to be concerned with power consumption. I believe there was a post earlier with a dedicated part form radio shack that is far better then a resistor voltage divider.

If you are trying to put a 5 volt LED on a 12 volt source   like the others said it is dependent on the current draw,  but you will be safe with a 1K Ohm resistor.

If you are needing it for something other than an LED as KPierson said the 7805 is the way to go and the tab of the 7805 is the ground connection so if you need 1 amp of current  just mount it to the chassis of the vehicle and it will use the car as a heat sink.

If you need several amps of current you can parallel the regulators.  

No, it's not the same.  The Ford amp uses a low current input to monitor when it should and shouldn't be on.

The OP is trying to drive a load with a certain voltage.  In a voltage divider circuit the more you load the circuit the more the voltage supplied will drive.  Resistors should only be used to create reference voltages, and should never be used to create a voltage based power supply