always run rca's from the head unit to the amp for your bass box/subwoofer, if you splice into your cd changer cables your just gonna have problems, and to save a little moeny it is possible to connect a subwoofer to the standard e39 business head unit, i know this because i have done it myself, only costs a pound or so for a speaker cable - rca converter, you then just splice into the cables for 2 of the speakers and hey presto, rca output from the standard head unit, i believe the required part is available from maplins aswell, hope it helps, drop me a line if you need help/advice with getting it working Rhys