I haven't pulled mine apart yet, but... I, oddly, just had to have the driver's side mirror replaced via an insurance claim. The body shop left the old parts in the back seat of my truck. There is a "star" (I have no better word to describe it) shaped retainer on the shaft of the hinge that captures the spring that provides tension the hinge. That's the only real mechanics of the hinge assembly. One or both of those parts must have failed. I wonder if someone makes a rebuild kit.