Back-up lights on in forward gears??

First question is "what model year" is the 904 out of. TFs before 69? 68? had the single terminal neutral switch, and while the 3 terminal switch is reported to work FOR NEUTRAL/ PARK, you can NOT use the backup light terminals on the older transmissions

If you have a later transmission, if you are SURE the wiring is OK, and if you are sure the switch is OK, then it's an internal problem in the trans.