I used this:
http://m.jaycar.co.nz/m_productView.asp?ID=AA3074
You have to earth the unit itself and I think, was a while ago now, you splice it into the constant power feed for the stereo. Once it was fitted the hiss or interference was gone.
Not a huge amount of room by the stereo, mine it tucked in underneath the switch blanks by the cigarette lighter.