CD changer would carry audio if you already have a CD changer, just splice into the wires. If the head unit doesn’t output audio when the cd player isn't playing, i guess you could burn an audio CD with just silence in it and set it on loop. Ugly hack but hey, It'd work! (I think the harness is on the right side of the trunk, but you have an E46 chassis and I haven’t had experience with these so I could very well be wrong.) I don't really recommend you do this, but it is possible and could work just fine.
Since you have a stock head unit, I doubt heavily that you will have an auxiliary input. I doubt you want to change out the head unit with an aftermarket one, so I'd go RF modulator. If you get a NICE one, the quality isn't really too bad. I'd avoid RF if you can find a hard-wired solution; otherwise, RF is the way to go for this application.