I had the problem with the passenger seat in my 81, I had to take the seat out and take apart the sides to figure it out. Turned out to be a small "square-ish" metal piece that was preventing it from moving forward, I could find no reason for it being there nor could I find what the part was for in my manuals. I haven't had any problem with the seat coming forward on its own but then again I have the 5-point harness in now too. Sorry this wasn't much help.