The kit is available and is a direct plug-in. THere is no need to tap into the harness. Only '03 models have the extra jack behind the radio. The input port is a stereo connector and does not seem to be an RCA. Not sure how an RCA would work as an Aux-IN? This looks like a real easy install and really cheap for a BMW acc. Here is a link of a thread discussing it and pics.
'03 E46 Aux-In harness
'03 E46 Aux-In harness