so you're going from computer to sound card to factory head unit?
If so, then the cleansweep might help you. It's all dependent upon whether or not the factory head unit does something to the sound coming from the sound card, like adjust the eq's for example. If so, the cleansweep will balance that signal for you. That's the typical application.. However, since you have a computer, you can actually do the balancing yourself using software eq adjustments. You will need a mic and a program like
trueRTA to help you balance you EQ levels. Then from there, make sure you only adjust the volume using your carputer.