This is a follow-up question. Here is my original question and the answer provided by Benny (expert currently maxed out):
"I'm trying to install a bath fan/light combo in an older home that was built in the mid-fifties. My plan was to replace an existing light fixture with the new unit. The light fixture was controlled by a double rocker switch (one rocker controlled the light that I replaced, the other controlled a light in the adjacent room). When I got to the junction box in the attic, I found that there was four runs of 12-2 w/o ground cable running into it; I assume one run for the light fixture I attempted to replace, one for the light in the adjacent room the switch also controlled, one for the light in a nearby bathroom and of course a run for the switch itself. I labeled each strand before cutting them and installing the fan unit. The fan had four wires coming from the back of it, 2 white and one black and one blue. After installation of the fan and connecting all of the wires (all whites connected to whites, blacks connected together with the blue wire as well) I found that the fan and the lights worked, but were not controllable from the switch. They run constantly unless I trip the breaker. Help! I feel like I'm in over my head.
Back in those days it was common practice to install the power wires to the fixture instead of the switch. From the light they would install a conductor (one black and one white) down to the switch. One of the conductors would take to power to the switch. The other conductor would return it back to the light after the switch was turned on.
You need to find out which conductor goes to the switch and hook the black wire to the hot conductor in the light box. Hook the black and blue wire to the other conductor that brings power back up from the switch.
If you copy and paste this link, the diagram might help more than I can explain.
I have identified the wire leg that runs to the switch, and wired it per Benny's suggestion and the wiring diagram provided. When the switch controlling the light/fan combo is in the on position, and the circuit breaker is thrown on, the light/fan come on as they should. All other lights in the circuit also work. However, when the switch controlling the light/fan is turned off, the breaker trips and kills the circuit. I have tried replacing the breaker, as well as replacing the switch itself (as mentioned before, it's a double rocker switch). If the switch is off to begin with, the breaker can't be pushed on. Googling this issue and checking previous All Experts questions makes it sound like there is a short, but all my connections are crimped tight and well wrapped. Can this issue have anything to do with the home and fixture being ungrounded? I can run a ground wire to a nearby galvanized cold water pipe if necessary, but I'm not sure if this will resolve the problem.