Connect your PC or other audio device directly to an amplifier, bypassing the head unit altogether.
If you have an external amplifier in your vehicle, you may not have to adapt your head unit to get audio from external devices to play through your vehicle's speakers. Although the head unit/auxiliary-in approach in "Get Computer Audio into Your Head Unit" [Hack #14] gives you the ability to control the volume of all devices from the head unit, you can connect computers and portable devices with their own volume controls directly to the RCA inputs of your amplifier, bypassing the head unit altogether.
Amplifiers come in two-channel (stereo) and four-channel varieties, with a corresponding number of RCA inputs. You can see an amp and these inputs in Figure 2-8. Some amplifiers also have a set of "high inputs" that can take the output of a cheap head unit's built-in amplifier, convert it down to a lower level, and then reamplify it with the strength and audio fidelity of the amplifier. There are also separate line-level converters that can achieve the same function, for when your head unit doesn't have RCA outputs.
If you install a separate amplifier for all your speakers, as in Figure 2-9, you can improve the sound quality and add additional options for inputting audio from sources other than the head unit.
Figure 2-8. An amp and its ...