I kind of left something out on accident for those of you who decide to do the guitar knob thing. You want to find the knobs that have a set screw and don't just slide on. The hole on the back of the knob is 6mm and most of the switch shafts are too small to just tighten up to. When I was looking for my wiper delay setup out of a early-mid 80's Ford truck, I found this cool little insert to hold the factory knob in place.
I looked around to find 3 more (it's only on the trucks and ONLY the wiper knob) then I cut the little top loop off and it fits perfectly onto the switches and inside the guitar knob!