reversing phase in pickup

[hensonbass]

I would like to combine a magnetic pu with my piezo setup and I need to reverse the phase on one of them to make it sound better.

How do you do this without a the use of a phase reversal switch on an amp? Is it a wiring issue?

[Francois Blais]

Indeed.
Just reverse the two signal wires on one of them.

[mje]

Since most pickups have an unbalanced feed (coaxial cable) with the shield of the cable connected to the internal shielding of the pickup or other grounded part, you generally can't simply reverse the leads without compromising the shielding or causing majhor hum problems. It very much depends on the specific pickup.

[fdeck]

There's a chance that you can simply flip the piezo upside down.