Along with the rear bulb, check your ignition circuit from the key switch(if it has one) to the handlebar "on/off" switch. Quite possibly, the moisture has contaminated an already rusty connection that is not making contact.
Also, I think there are two wires, a red and a black one, that connect to the top of the motor, one on each side. Check them for security. You can access them by removing the "hood".