When Leo Fender was designing the Strat he decided that because certain other guitars (like the Les Paul,) had two pickups his new model would have three to go one better.
Firstly, most guitar tone controls are simply a shunt for the treble and give a muted sound which many players simply don't like or use so the controls stay fully up. The first modification is to replace the controls with three volumes, this way when any two pickups are selected together the sound may be mixed using the individual volume controls.
Second, the center pickup may be wired "Out of phase" and this gives a terrific sound whenever the combination of neck + middle or middle + bridge is selected. To retain the option of still getting that 60s blues sound the mid pickup may be switched in or out of phase. Also by mixing the volumes of the two pickups which are in circuit the variation of just how far in or out of phase you require is infinite.
Third, a bleed capacitor may be added to emulate the sound of the Telecaster this again can be in or out of circuit depending on the particular sound you require.
Fourth, many early 50s/60s guitars did not use screened cable internally and they had a certain sound, which many people try to get back to. The downside of not using screened cable is that you are likely to pickup hum and general noise from lights etc. so modern guitars are generally fitted with screened wire. A better method is to go back to the original design of using heavy gauge non-screened cable and to screen the internal cavity with nickel screening to prevent hum pickup. This way the treble and harmonics are retained but hum is reduced.
All the above modifications are achieved without any drilling or altering of the look of the instrument and without the use of batteries.