I was thinking about this today and it sounds like a great idea. Basically I was thinking, on my car stereo, a pioneer DEH-P400, there are 3 modes or functions: CD player, radio and M-CD, which I guess is for a multiple cd player carousel you stick in the trunk. I don't have one of those, but would it be possible so simply solder a few wires together so I could input from my mp3 player into the M-CD channel?

Does anyone have experience with car stereo wiring? I no longer have the manual and it's hard to find any info on an older model car stereo, so I think I'll have to take it apart this weekend and have a look at it. If it could work, it would be a sweet mod!

I'm sure godfoot or another one of our pimpin' audiophiles would be able to help you better, but in the meantime I found something for you:
http://parts.pioneerelectronics.com/...delNum=DEHP400
You can purchase a service manual for your model stereo, but it's almost $30. I tried searching for a .pdf or equivalent, but to no avail. Maybe someone else will have better luck.

CNET guide to hooking up an MP3 player in your car - http://reviews.cnet.com/4520-10895_7-6262887-3.html

Actually your Pioneer has a small white plug receptacle on the back usually on the side of the antenna...
That is where you plug in the accesory wire harness that you can purchas...It allows you to connect anything with rca jacks....
http://cgi.ebay.com/AUXILARY-ADAPTER...QQcmdZViewItem

However..I once modified my Pioneer deck to do this without the wire...

take an old pair of junk headphones and cut the wire off....
Seperate the 4 wires you find....
Usually two wires in each wire you find two internal wires....One is green white the other is red white....
Get some small pin plugs at radio shack and crimp them on the wires...don't forget insulation...

Then plug them in to I believe the four outer wires in that plug....You will have to experiment to get it right as I don't have the diagram handy anymore...I simply just put my stereo balance to left...and used the wire set I made for the left channel. Plugging it in to the diff pins till I heard sound...Switch balance channels and repeat....Then just fasten the wires properly so they dont come out...
That is of course the ghetto way to do it...I have since just purchased the adapter that Pioneer sells for that plug....


Digi

alot of decks like my jvc have a damned proprietary connector that allows you to use anything with RCA jacks like digital world points out. they are expensive usually ($40 for the one for my deck plus shipping since no one local would have such a dongle)
for the cd changer yes you can make it so it just gets audio but you will need to get a harness which probably will cost as much as an aux in cable. (praise companies like sony that have RCA jacks on the back)
you could also open the thing up and add wires to where the connector would go. ive thought of doing it but its something i dont want to mess with since its expensive and i wont be needing a car soon.
a RF modulator is ok depening on how good of reception you can get from it. though i reccomend the dongle if your deck can accept one since you wont be messing inside something that probably cost more than most computer components that we cut up.

Digital World, dude thanks for that tip! I bought one of those connectors today and it really was that simple, just plugged it in and I now have MP3 output in my car, instantly making radio and CDs obsolete!

Perhaps the best upgrade I've ever bought for my car.