no AUX button...

Well, I took off my DHD head unit (POS) that came with my car and I wanted to connect my MP3 player to it.. I have done it to my other car so I know what Im doing... the question is.. There is no AUX button so do I hold a certain button to go into AUX mode or something b.c there are RCA inputs on the back of the head unit so I would assume its compatible... . btw, Im using a "Y" adapter with RCA red and white ends and on the other end.. the headphone jack.. thanks.

most of the RCA plugs on headunits are outputs, you need to read the user manual.

yeah, if there's not a button on the face that says AUX or something, it's 99% likely that your unit doesn't have an aux in. Most lower end decks don't have that feature. My suggestion is to get a new deck, one with either aux-in or MP3 CD (or even better DVD) capabilites. If you've already got an iPod or whatever, Walmart has a couple deck models with aux in for $70ish. Some of Alpines and other manufacturers have highend units with both DVD and iPod control. Depends on what you have to spend.