Most head units accept the AUX input through the CD changer input. So you will have to get the cable which plugs into the head unit's CD changer input (usually a DIN head) and the other end can be either RCA or stereo plug.
I got my Alpine AUX in cable for RM80 (ordered from USA). The local distributor here sells it for RM120 (or RM150), somewhere around that price range.
Of course, if the AUX in is RCA/stereo, then super cheap lah.
Another way is to have a stereo-RCA cable, plug ur mp3 player into the stereo end, and the RCA end, plug it into a pre-amp or directly into the amp (but kinda whacky since the iPod doesn't have a customisable EQ).