There is a 12 volt + and - terminal on the speakers for the illumination (don't confuse with audio + and - terminals).
Connect the - to chassis ground.
For the + terminal, you have several choices:
1. Connect the + to the remote output terminal from the head unit so the illumination is on only when the head unit is on.
2. Connect the + to one side of an on/off toggle switch and the other end to constant 12 volts (with 5 amp fuse) so you can turn on or off when you want (becarefull not to forget to turn off).
3. Connect the + to one end of an on/off toggle switch and the other end to an accessory switched 12 volt source (with 5 amp fuse) so you can turn the illumination on and off when the vehicle ignition is on only.