Here is the wiring diagram for your heater.
Your description of the rocker switch seems to indicate you may have two separate locations for turning on the electric element as the wiring diagram doesn't match your description. Regardless, you do need 12v power to the board through the white wire, and from there, the yellow wire on the relay should power up and activate the relay.
Be sure the white wire that connects to the board has continuity
to the white wire you find at your switch. The diagram is simplified, but the path of 12v will be the same.