I got the bits I ordered last night, so I sat down and started playing with them. The LED is pretty bright, so I might throw a higher value resistor in series with it. No use in using too much power if you don't need to, right?
Also, the photoresistor didn't work exactly as I had thought, so I had to tweak the resistor in series with it to get the photoresistor. It is now a 3.8K resistor instead of a 500k. I'll make a new schematic once I get things mounted and working in the car. That resistor may yet change again as might other things.
Unfortunately, with the change to include the clutch switch signal, I'm going to have to order more parts since I don't have a 5V regulator or any 6v zener diodes laying around. I also still have to figure out if the zener diode thing will even work...