Sony CFD-ZW755. I originally wanted to disable only one tape deck. This would have required using pickup head wiring. I decided the quality of sound from cassettes and the few I had left meant I could sacrifice all cassette players to have a 1/8" input for an MP3 player or smart phone.
A common pre amp for dual or single cassette decks in the Toshiba TA2068N. In my setup I wired the 1/8 aux 3 wire plug to the outputs of the TA2068N. I used the output pins of 1 for (-) and 18, 20 for (+). There is a resistor and capacitor on the pre-amp board that I wired my input through to retain input stability to the main amp.
I drilled a hole and mounted the 3 wire 1/8 input. I also soldered the tape input switch for #1 tape player and disconnected the leads to the tape player motor. There is no simple mode selector switch in this unit. It is all electronically selected.
Now I just select Tape (for aux input) plug in an MP3 player, or in this case my Moto E phone. Volume on phone has to be around 75% to 85% volume and the audio is very good.