It would seem that if there is a short it would be on one of the pins 1,3,5,7 on S1. Since the glow plugs won't trip the breaker, then pin 1 is likely OK. So, if you disconnect pins 3,5,7 on S1 and turn s1 on you can see if CB1 trips. If it doesn't, then connect pins 3,5,7 one at a time and see...