I don't follow what the momentary switch is doing, you still have to press the switch to activate it.

Why not use a normally closed relay that is toggled off the ignition power line. So the switched side of the relay would control the cam position sensor, and the signal side is from your ignition power. This way when there is no power applied to the ignition line (such as during cranking), the relay will be closed and allow the Cam Position Sensor to work. As soon as you are done cranking and move the key back to the Run position, it will open the relay and cut power to the Cam Position Sensor.

If you need it to stay on for a few seconds after cranking, you can just use an On-Delay relay such as this one:

http://www.mcmaster.com/#77055k733/=th70a1

Although you may want to see if you can find one more suited to an automotive use, unless you are willing to mount that somewhere in the cab, and just run a long wire to the Cam Position Sensor.

This way it is all automatic, no momentary or latching switches to deal with.