where is the stop and tail light check unit located.

Failed/open fuses can easily be overlooked while checking .. A lot of vehicles have a designated fuse for each of the rear 'tail' lights .. Just saying ...... Using the same test light that was use to confirm that ALL fuses were indeed ok - with the park lights ON at the time (?) .. Test for battery power for the tail light at the tail light wire harness connector, if no power is present, start checking harness toward the front of vehicle until power is detected.... Interior trim panel removal may be required... Note: 'IF' wired for trailer towing, start testing there! (Occam's Razor)