I just thought of this (thanks to the RF interface suggestion): If the aftermarket CD player has CD changer control capability, there may be an interface (either from the player manufacturer, or third party) to add aux-in capability via the CD interface. I briefly considered this as it seems to exist for the later '90s Volvo factory radios (SR-xx models, I think). But I'd still need to buy another factory radio, and the interface isn't cheap, either. But it may be for the aftermarket radio.