348 seat belt problem

My husband is obsessing about fixing his 1994 348 Challenge seat belt on the driver's side, which apparently was disconnected at some point. He has the seats out but does not know where to go from there. (The one on my passenger side works.)
Any advice would be much appreciated. He wants the car to be in its original condition and perfect. We have a wiring diagram, I think, but are not very swift about the electrical system.
He is a computer illiterate, and now ticked with me for not researching this sooner - so our marriage can use some support! - Nancy