Headlight and Reverse Lights monitor for YouTube Commenters - UNTESTED but should work fine.

LightMonitor.py

#!/usr/bin/python
# A quick hack of my button monitor script to monitor headlights and reverse lights to 
# Dim the screen and switch to the reverse camera.
# I have not tested this...
# Remember, the switch or opto is switching to ground, so the logic is inverted... 0 = on  1 = off
# Also, you would run only one of these "Monitor" scripts. you can't be running RearviewMonitor.py AND LightMonitor.py

import RPi.GPIO as GPIO
import time
import subprocess, os
import signal
GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)
RearView_Switch = 14  # pin 18   (You must use an optocoupler and resistor as shown in the video)
Brightness_Switch = 15 # pin 16   (You must use an optocoupler and resistor as shown in the video)
#Extra_Switch = 1  # pin 3
GPIO.setup(RearView_Switch,GPIO.IN, pull_up_down=GPIO.PUD_UP)
GPIO.setup(Brightness_Switch,GPIO.IN, pull_up_down=GPIO.PUD_UP)

print "  Press Ctrl & C to Quit"

try:
    
   run = 0
   bright = 0
   while True :
      	time.sleep(0.25) #this restricts the script to check the lights every 1/4 second, There is no point in checking 10,000 times a second.
	      # If the reverse reverse lights come on, do this:
      	if GPIO.input(RearView_Switch)==0 and run == 0:
         	print "Switching Rearview Camera On"
         	rpistr = "raspivid -t 0 -vf -h 480 -w 800"
         	p=subprocess.Popen(rpistr,shell=True, preexec_fn=os.setsid)
         	run = 1
        # When the reverse lights go off, do this:
      	if GPIO.input(RearView_Switch)==1 and run == 1:
         	os.killpg(p.pid, signal.SIGTERM)
		      print "Killing the reverse camera feed"
         	run = 0
          
	#These next Two blocks monitor the headlights or marker light and adjust the screen brightness settings.
      	if GPIO.input(Brightness_Switch)==0 and bright == 0:
         	print "Setting Brightness to 20" # 20 is about 10%
         	subprocess.call ("/usr/local/bin/backlight.sh 20", shell=True)
         	bright = 1
         	
      	if GPIO.input(Brightness_Switch)==1 and bright == 1:
         	print "Setting Brightness back to 255" #255 is 100%
         	subprocess.call ("/usr/local/bin/backlight.sh 255", shell=True)
         	bright = 0
       
	
except KeyboardInterrupt:
  print "  Quit"
  GPIO.cleanup() 

This works as deigned. I have a video of it running on youtube if you are interested. https://youtu.be/PYI-8KDzQV8

Print "Killing the reverse camera feed" was giving an error. I removed it and all was well.