Pioneer - Unpin the mute wire and relocate it to the opposing corner of the harness. Ground both the mute and parking brake wires. There's also a way to do it via software.
Kenwood - Ground the P.Brake wire.
Jensen/Phase Linear - Ground P.Brake wire OR 543210 on the remote, and change the setting. Have to get the speed right on the 543210. Only verified on the flip outs, never tried it on a double DIN.
Dual - Ground the P.Brake.
Alpine - Get a PAC TR7, and **** the bullshit.