The front seat in the Buick is a 6-way power unit. The control knob on these seats is made of plastic. It’s pretty tough, but apparently not tough enough. The original knob is busted off. As luck would have it, an eBay search turned up a NOS replacement unit, still in the original Buick Parts box! For such a rare part, I think it’s not a bad buy at $36.

6-way power seat control knob