I'm pretty sure the power source is coming from the light between the 3 way switches. I have a black, red, and white attached to the switch. I have a black & white, (I believe coming from the light) The black goes to the switch. The white is wire nutted to a Black (I believe is coming from the other switch). I can get the outlet to work only when the switch is in the on position. Is it possible to wire it to have continuous power?