Shopping from Canada? 
Go to Defender Canada!
Dexter
Dexter Stainless Steel Reverse Lock Out Solenoid - 81103
Item: #1872079 | Part Number: 81103$80.99
Description
Unlike drum brakes, disc brakes generate full braking power in reverse. If your actuator does not have a reverse lock out lever or switch, a reverse lockout solenoid should be installed at the actuator before installation of disc brakes. Only one electrical connection required between solenoid and vehicle backup light....
Dexter Stainless Steel Reverse Lock Out Solenoid - 81103 Item # 1872079
Description
Unlike drum brakes, disc brakes generate full braking power in reverse. If your actuator does not have a reverse lock out lever or switch, a reverse lockout solenoid should be installed at the actuator before installation of disc brakes. Only one electrical connection required between solenoid and vehicle backup light. Can be used on drum brakes as well. Sold each.