Remove the back-up light switch (A). Check for continuity between the back-up light switch 2P connector terminals No. 1 and No. 2. There should be continuity when the switch end (B) is pressed, and no continuity when the switch end is released. If the result is faulty, replace the back-up light switch. If the switch is OK, check the reverse shift piece in the transmission.
Apply liquid gasket (08C70-K0230M, 08C70-K0334M, or 08C70-X0331S) to the threads of the back-up light switch, and install it on the transmission housing.