Hello, I am using the Pioneer DEH-2900MP(the red version with no aux-in on the front) radio in my car. It has what looks like an aux-in on the back,I really want to be able to use my iPod to play music through the radio, how do I do it?

SOURCE: aux

there is an interconnector to connect this unit to the aux source (CD-RB20/CD-RB10)(sold seperately).
After connecting, press SOURCE until AUX appears in the display.
Press "play" on your ipod.
i sincerely hope this answers your question.

SOURCE: connect ipod to cd player wit built in aux input.

i have the same problem with my kenwood cd player.....only gives me 3 options cd,tuner,standby

SOURCE: how do i turn aux on my pioneer deh-7600mp radio

Shut it off then hold the function button, cycle through till you find your option to turn it on, the turn the radio back on and use you source button to change to CD, TUNER, and AUX

SOURCE: ipod worked with my pioneer deh-p3700mp before

this works on some other pioneer head units i encountered it may work on yours.
Shut it off and hold the function button until the options appear then you can scroll through and change a bunch of settings including the aux input

SOURCE: Hook an ipod to Pioneer DEH-16

The RCA jacks on the rear of the head unit are rear pre-amp outputs. (google it if you need an explination on what purpose they serve or see the installation guide) With that said, I am able to play my mp3 player through them but since they are 'rear' pre-amp outputs my mp3s only play on the rear speakers. see this post for more...
http://forums.creative.com/creativelabs/board/message?board.id=dap&message.id=45149&query.id=194369#M45149

installation manual:
http://www.pioneerelectronics.com/pio/pe/images/portal/cit_11221/35572505Installation%20Manual%20DEH-1600.pdf

owners manual:
http://www.pioneerelectronics.com/pio/pe/images/portal/cit_11221/
33399012operation%20manual%20deh-1600.pdf