2003 Classic Prius Audio Question Help

I really need some help here. I Have a 2003 (Classic) Prius and want to add RCA Aux audio inputs for adding mp3 devices. I have read everything I could find regarding this, but the available adapters I find either don't work, work spotty or have problems by members, etc. The ones that seem to work like the adapters at Coastaltech seem to be sold out.

I have a AM/FM cassette with a single CD player. I am not interested in going with a tape adapter or a FM modulator.

Can anyone steer me the correct way to find an adapter I can use for this purpose? Thanks ahead of time.