you would be looking at a full logic board replacement. The haptic sensation from the second-stage push-down on the trigger occurs when the trigger assembly places pressure (from your finger) on an embedded metal tensile pad which will click when enough force is applied to it. There's no feasible way of removing and replacing the sensor as it's metal appendages are folded into the PCB board.
Your best bet would be to leave the warranty stickers (covering the three internal PCB screws) in tact and contacting steam customer service, otherwise you're probably looking at a new piece of hardware.
rip.