I am no expert -- but I have a '74 Challenger and according to my Service Manual, that looks like the "Interlock Unit" that has two connectors coming into it (a 9 wire or cavity and a 7 wire). As noted, it is part of the '74 only interlocking seat belt system (I still have mine in my car, but I disconnected the switch).... If any of the info in this manual can be helpful, let me know and I'll scan it for you (probably not if you are simply going to by-pass it).
Just curious -- does yours still have the "reset" switch on the driver's side fender wall - just above the battery (about 2 inch rectangle box with red "button."
VT MOPAR