Access to vehicle audio from standard headphone jack input

My apologies for the extreme generality of this, but I wanted to move forward with a little project of forking my audio off to have access to a stereo head-phone jack in addition to its normal speaker operation. I understand how to do the wiring part (though comments on this are appreciated), but am at a loss as to where I might find a good place to split off along with other things I need to consider.

The vehicle is a 2002 Mazda Tribute with stock equipment.

The purpose: to have the ability to jack into the vehicles speaker system with a portable MP3 player. Maybe there is an alternative (smarter) way to do this?

Looking forward to the responses, thanks for the help.

interfacing aftermarket equipment with OEM stuff is always a challenge at best. I am not sure what you really want to do, are you wanting to put headphones on your stock system or use the headphone output of an ipod type device to play thru the speakers ?

use the headphone output of an ipod type device to play thru the speakers ?

This.

I doubt an ipod will have the power to drive 4 car speakers, what most people do is get a headunit that has an ipod port OR some kind of box on the output of the headunit that will switch between the two. Some people have also tapped into the DIN plug that is meant for the changer or the satellite radio - this might be a little more difficult of a project.

I doubt an ipod will have the power to drive 4 car speakers
Maybe I'm missing something, but what would the mp3 device have to do with powering speakers? I'm just looking to fork off the audio to a allow for additional input.

ok, I meant to get back to this earlier ...

You have two basic options, you can either get a headunit that has this type of input or splice into the speakers wires at the output of the radio and add it there. If you choose option 2, you WILL be driving the car's stock speakers with the output of the ipod. As stated above, I seriously doubt you will get any volume that way, those outputs are preamp/headphone outputs. The only other option is to get a preamp with a 2 inputs and a 4 channel amp - this is the expensive route. I don't think there are any other ways short of taking the headunit apart, finding the preamp inputs and making some kind of connection there.

I had figured the speakers would have power from another source and sending an input would be as simple as splitting the audio cables.

Thanks for the insight on this; I've got some direction at least.