I just dropped a Seymour Duncan-loaded pickguard into my Strat. The pickguard has a push/pull Tone knob. I understand that the knob is supposed to activate the Bridge and Middle pickups when the selector switch is in position #4, and all three pickups when the selector switch is in position #5. However, when I pull the knob with the selector switch in either of those two positions, nothing happens. Zip. Nada. Ideas? Anyone?