OEM radio - and ipod/mp3 input?

I've got the stock CD/Cassette combo. The PO added a Tsunami output box in to feed an amp.

I'm like to add an mp3 player. Is there an external input available for this radio?

I've considered an fm converter, but am not convinced of their sound quality. Cassette adapters cut off the bass response at 100Hz which excludes a lot of bass information.
I'd like to keep the cassette player for audio books from the library.

Thanks.

Does the factory CD/tape combo have cd changer controls? I know on some Sonys and Alpines, you can trick the unit to think a changer is connected and use the bus audio in(you'd need a wire diagram to know which of the changer wires is the changer output/audio input to the head unit and which one is the changer connectivity recognition). As you know, the easiest solutions are the two you're against. Use the fm modulator or install a different head unit.

I don't believe there are any CD changer controls. I was hoping there was a pin dedicated to this, though.

I serious doubt the oem radio has the pins, otherwise they'd advertise the feature. You have an amp in the system, why no plumb the ipod output to that? You could get a stereo jack that disconnects the radio head and patches in the ipod.

The FM transmitters that expect to transmit a signal outside the vehicle to the antenna are really bad as you said, but have you considered the type that goes inline in the radio antenna wire?

http://www.crutchfield.com/p_142FMMOD02/Scosche-FM-MOD02.html?tp=121

I haven't used this particular one, but I have used similar ones and the sound quality is head and shoulders above the little transmitters and the cassette adapters.