Strats are great guitars for hot-rod electronics work. It's easy to disassemble them and get at their guts without poking into a little cavity, and there are lot's of cool ways to rewire them.
This modification is going to solve a couple of complaints I've always had with Strat electronics. First, I've always wanted a tone control that controlled every setting, not just one control for the neck pickup and another for the middle. Second, I've always wanted to get the two outside pickups (neck and bridge) for more of a Tele-type tone. And why not see what all three pickups sound like at once?
First, let's get acquainted with standard Strat wiring. Take a look at Diagram #1. This is what you should see when you disassemble your Strat and flip the pickguard over. I've coded the tabs of the pickup selector switch as follows: C = Common, N = Neck, M = Middle, and B = Bridge. If you've got an axe with a bridge and/or neck humbucker such as Ibanez or Jackson guitars, your selector switch may be wired quite differently and youre not going to be able to tackle this project. Sorry. Also, if your pickup switch is one of those dinky import models encased in plastic, I'd suggest getting a good old-fashioned switch and making the change. The dinky plastic units melt and warp with multiple solderings.
And be aware of really kinky import switches with weird leaf-spring connectors. These are worth replacing, too. If everything looks pretty cool, but your pickup selector switch looks flip-flopped or the common connections are in different places, that's no problem as long as you understand where the signal is going. If your axe doesnt have the ground wires connecting the casings of the various pots, add them. Some manufacturers rely only on the ground connection via the silver foil on the pickguard and this can come loose or oxidize eventually.
Assuming you know how to solder, youve got some spare 22 or 24 gauge wire around, and youre not going to rewire a valuable 63 Fiesta Red Fender Stratocaster, lets go to work.
Step One: A Democratic Tone Control
Locate the wire that runs from the neck tab to the tone side of the pickup selector switch to the middle tab of the neck tone control. Unsolder this wire at the neck tab. This liberates your neck tone control. While you're at the pickup selector switch, unsolder the other two connections on the tone side (the Middle tab and the Common tab) and carefully unsolder the Common tab of the pickup side of the switch and remove only the wire that connects the two Common tabs. If this wire turns out to be part of the same wire that runs to the volume control, them neatly snip it off as close to the tab as you can get while keeping the volume-common connection intact. Notice, by the way, that the tone side of the pickup selector switch is now free of all wires and unemployed for the moment.
To turn your neck tone control into a dedicated, democratic tone control, first remove the wire that runs between the neck and middle tone controls. Your middle tone control is now nothing but a potentiometer, full of potential but currently unemployed, along with half of the pickup selector switch. Now, take that long wire that goes to the center tab of the neck tone control and resolder it to the center tab of the volume control. Your neck tone control is now a master tone control, able to roll highs off of any pickup configuration you choose.
Testing your work at this point is easy. Plug your disassembled guitar into an amp at fairly low volume. Turn the guitars volume full up. In each position of the pickup selector switch, tap the pickups with a screwdriver while rotating the tone control. Every appropriate pickup should yield a sound that varies from a bright click to a dull thump.
Step Two: The Big Switch
Now, let's put those unemployed parts to work. First, let's get the tone side of the pickup selector switch back in action. Run a very short wire from the neck tab of the tone side to the bridge tab of the pickup side and solder it in place. Now run a wire from the bridge tab of the tone side to the neck tab of the pickup side. Be sure this wire is insulated, because it crosses right over the other wire you just added. Solder it in place, making sure you dont melt the insulation. The tone side of your selector switch is now accessing your bridge pickups output in the neck position and your neck pickups output in the bridge position.
Finally, we're going to turn the unemployed middle tone control into a volume control for those extra settings we just accessed. Unsolder the two wires that are still attached to the middle tone control. Run a wire from the common tab of the tone side of the pickup selector switch to the middle tab of the middle tone control and solder it in place. Maybe you can use the long wire you just detached for this. Run a short wire from the previously unused tab of the middle tone control to the middle tab of the neck tone control. To finish off the conversion, run a short wire from the casing of the middle tone control to the only tab on it that's not in use. You might be able to bend this tab up to contact the casing directly, but be careful. The last thing you want to do is break off the tab and ruin all the good work you've done so far. Solder this connection up and check your work.
Now, lets test your work before we stuff the spaghetti back into your guitar. Plug your disassembled guitar in and set your pickup selector for the neck position. With your master volume full up, you should get a normal clunk when you tap it with a screwdriver. Now roll your middle tone control to ten. You should get a clunk from your bridge pickup now, too! Rolling back the control rolls back the volume only on the extra pickup. Cool! Flip to the bridge setting and see if your neck pickup is fading in and out in the appropriate way. Double cool! Now try the double-pickup positions and see if the third pickup fades in and out. Triple cool!
Reassemble your new creation and plug in!