http://www.mp3-box.org/mp3-car-stereo-connection/
Car- Mp3 player – connectivity! what are my options?
I have a 97 nissan sentra and do have a tape player unfortunatly it does not work ! 
. Anyways i know i could get a fm transmitter but i heard they have there draw backs as far as reception goes. I know i could connect it directly using RCA connections or a Aux input the only problem is I dont know how to do the wiring in the back, or what connections will i need? I dont want to hassle with stereo experts cuz they charge a whole bunch. So if theres anyone who has a good idea or is familiar with this car stereo model please help.
the wiring is simple
your old tape player has the wiring drawn on it
and the mp3 player should too
the colors of wires can be different but use the picture they provide to connect the wires
Sony Xplod Cdxgt42Ipw / Cdx-Gt42Ipw Mp3 Car Stereo (Cdx-Gt420Ip)