How can I determine where power enters my bathroom light circuit?
OP seems to have done a scarper but for anyone else reading this: 

The one sure way to find where the power enters a circuit is to completely disconnect each cable and drop a test load across its wires. 

I detach the cables one at a time and re-connect each one before going on to the next. This saves having to mark everything first -- although it does involve a lot of running back and forth to the circuit breaker panel. 

My favorite test load is a neon indicator lamp on a short piece of lamp cord with two insulated alligator clips. But almost any kind of lamp can be pressed into service.