In my small Trailer I have three 12 volt circuits coming from the power center. One circuit blows the fuse as soon as the battery is connected. I thought I had a short somewhere in this circuit so a placed a test light in place of the fuse and thought that with everything on that circuit shut off or disconnected the test light would be ON to show a short and would go out when the short was cleared. However the light only goes ON when I turn on any switch in the circuit. With the switch on the test light shines but the light or pump on that switch still does not work. Why doesen't anything work??

You are correct, when you have light in place of fuse, and it is out, there are no shorts up to the switch, the short, or excessive load is after the switch.
I would remove bulb, turn switch on and see if it trips, if it does, short is between switch and light. Remove bulb and try again. Also see if you can disconnect close to pump, to determine if it is the pump or wiring between pump and switch. Any recent work done?
Carpentry? Installation of something that could pinch wires?

Statmando: Thanks, Today after 3 days of tearing everything apart trying to find a short, and after seeing that the test light did not go on, I put the fuse back in and everything worked. There must be something somewhere that got jiggled and removed what short there was. Now I have no way of finding what it was.