I don't have an aux in option on my H/U.
Using 2 into 1 RCA "y adaptors" to connect both carpc (MII12000) & Alpine H/U (line level output) into my amp (4 channel) is causing problems. With just the carPC connected to the amp audio is great, until you connect H/U input (also line level) into y adaptor simultaneously, in which case volume drops dramatically and interference is audible (even where the h/u is switched off). Vice versa is also true.
I've searched the forum and have found this should not be a problem (eg.
http://www.mp3car.com/vbulletin/showthread.php?t=28362).
Only option I can think of is to connect the H/U into the CarPC line input and only run one source into the amp however this may have its own issues???