If you have a guitar with three pickups like a Fender Stratocaster but would rather use a five-way rotary switch, you can add one by simply wiring it the same way as the original switch. The electronics involved with a rotary switch are the same as the standard kind; the difference lies in the use of a turnable knob to switch pickup configurations rather than a traditional switch.
Solder the bridge pickup wire to the stage-one post. The stage-one area contains the posts for the pickups, and they must all be wired to this stage for the rotary switch to work correctly. The rotary switch works the same as a standard five-position Stratocaster switch seen on Fender guitars. The rotary switch looks like a volume or tone knob and should be used in place of the blend knob on your guitar.
Wire and solder the middle pickup to the next post, the two position in the stage-one area, which will be right next to the first position. This will allow you to use the middle pickup by itself or in conjunction with the other two pickups.
Solder the third pickup for the neck in the third position just as you did the first two pickups. The pickups will now all be wired to this input stage and you will use corresponding positions on stage two for your switching.
Solder an individual wire from post A on stage one to post A of stage two. This will allow the signals from the pickups to be sent to your volume pot, where you can adjust the volume of all three pickups at the same time.
Connect and solder the stage-two number-two post to your middle pickup tone knob. This will allow you to use this tone knob to cut the high frequencies of your middle pickup.
Connect and solder the stage-two number-three post to your neck pickup tone knob. This will allow you to use this tone knob to cut the high frequencies of your middle pickup. Bridge pickups usually do not have tone knobs because this is the commonly used pickup for rhythm guitar, which is usually not adjusted on guitars. But if you have an extra spot for a third tone, wire that to the number-one post for the bridge pickup.
Slide the post of the switch through the hole in the control cavity where your blend knob used to be and install the new knob by flipping the guitar over and securing the tiny screw in the knob to the post or simply by pushing it onto the post itself.