Here's something I have done before, with good results. Does it have a cassette player? You have to be good at soldering really small wires, though. If you think you can do this without ruining the radio (From static discharge), read on. Ok, inside the cassette playing mechanism is a read head. It's the thing that reads the change in magnetic field on the tape inside the cassette and sends the weak signal to be amplified inside the radio, then sent to the speakers. If you feel comfortable taking your HU apart, you will see that the head probably has 3 wires, a ground, a left channel read, and a right channel read. The ground typically is a shield, and the other two wires may be red and white, or red and black. What you need to do is trace these wires back to the PCB (Printed Circuit Board), and solder on your own wires that piggy back the read head wires. Shielded wires are preferable. You can purchase a decent quality rca- rca jack, cut the one end of rca jacks off, and drill a hole in the HU for the rca jack (You might not have to do this, if you can find space to route the rca wires outside). Once you have traced the wires back to the PCB, strip the wires for the RCA cord you bought, and join the two shielded wires together, and strip the red and white wires. All that is needed now is for you to solder these wires to the pcb where the read head is soldered. Shield- shield, red- red, white- black or white- white. Now, if you have a cassette deck that automatically goes to the other wide of the tape, to hear the auxillary, you have to switch yoru head unit to cassette (With no cassette in it!), and you may have to tell it to read the other side of the tape before your auxillary input is activated. There is also another complication that is possible, and that is certain decks sense when only one of the drive rotors is spinning (I.e. when the tape has ended, or when there is no tape in the deck) and they will switch to side B, back and forth from a to b. There is a way to disable this, but it's even more complicated (You cut the power to the drive motor, and wire it to a remote switch). Another thing to know is that the read head is very sensitive, the incoming signal must be pretty low, or it will distort. Like I said, I have done this a few times with good success, even making a small guitar amplifier out of a microcassette recorder once.