Am installing a garage door opener in home prewired (wires installed by builder behind finished walls). The red and green lights on the sensors blink continuously. I have tried adjusting alignment and even put sensors within inches of each other but still only get blinking. Prewire has three wires but since the manufacturer instructions only call for two wires I have used the same two color wires consistenly and have just left the third. Any advice would be very much appreciated. Thanks!