That depends on how you want the system to work. Read on.

First of all, let's clarify a few things. By "head unit", we mean any standard aftermarket car stereo, such as a CD player, a tape player, or a minidisc player. Integration with factory stereo systems is a separate (thorny) issue. So for this discussion, we're talking strictly about the daisy-chain order of the line-level audio outputs of various aftermarket components.

Also note that you don't need a separate head unit in order to use the Rio Car. If you install an amplifier in your car, the Rio Car can be your only stereo. That's how I have it, and it works great.

Finally, remember that you need room in the dash for both stereos if you want to do it this way. Not all cars have a double-DIN opening in their dashboards. So make sure you've got the space for it before you assume you'll be installing two stereos.

If you still want to use more than one head unit, there are three different ways to do it:

• You can plug the other head unit's outputs into the "Aux In" of the Rio Car.



• You can plug the Rio Car's outputs into the "Aux In" of your other head unit.



• You can route both devices into a line-level switcher such as the Sony XA-39II, and send both outputs to an external amplifier.

• The head unit must have line-level outputs. Most do, but check to be sure.



• You must have a separate amplifier for the Rio Car's outputs.



• You can use the Rio Car's incredible 20-band fully parametric equalizer (with separate front/rear options) to tailor the sound of your head unit, too.



• You will be using the Rio Car's remote control and front panel to adjust the volume, EQ, and other audio settings.



• The front-rear fading will be controlled by the Rio Car. You will use either the front or the rear outputs from the head unit and leave its fader centered.



• The Rio Car must be plugged into the dash in order for the system to make any sound at all, even if you just want to listen to the head unit's radio tuner or a CD.



• If the head unit has built-in speaker amplification, you can't use it- an external amp system is required.



• If the head unit has a special separate output just for a subwoofer, you can't use that, either (although such a thing is pretty easy to set up yourself if you need it).

• The head unit must have a line-level "Aux In". This is not always a common feature, so check to be sure.



• You can listen to the head unit if the Rio Car gets left at home. However, if the head unit is a removable-faceplate or pullout version, then you need the head unit to be plugged into the dash in order for the system to make any sound at all.



• If desired, you can use the built-in amplification that comes with the head unit (if any). This would allow you to skip the hassle of installing a separate amplifier.



• You can't use the Rio Car's equalizer on your head unit. If you need an equalizer for the head unit, you have to install a separate EQ unit.



• You will be using the head unit's remote control and front panel to adjust the volume and other audio settings.



• If the head unit has a separate subwoofer output, you can use it.



• The front-rear fading will be controlled by the head unit. You will use either the front or the rear outputs from the Rio Car and leave its fader centered.



• Some aftermarket head units with CD-changer controlling features might need to use the Aux Ins for that purpose, so check this out if you intend to integrate a CD changer into the system.

• The head unit must have both front and rear line-level outputs.



• A separate amplifier is required.



• You can listen to either unit, regardless of whether the other one is plugged into the dash or not. The switcher will automatically select the active unit and route its output to the amplifier.



• Each unit will have its own independent audio settings and controls.



• You won't be using the Rio Car's equalizer to tailor the sound of the other head unit. A separate EQ is needed if you want to EQ the head unit.