I had this when I did my connects2 (Aux in) install, especially when I plugged an audio/video source into the ciggy plug. I bought a 'Ground Loop Isolator' from Maplin, it was £8.99 and solved the problem.
http://www.maplin.co.uk/Search.aspx?criteria=ground%20loop&source=15&SD=Y
Hope this helps.
Looking good so far, post some pics of the finished setup ger.