I sure hope this isn't a completely stupid question.

I have a Pioneer DEH-3370XM connected in a SIMPLE 4 speaker (2 front, 2 rear) setup. No additional amps, no subwoofer, etc.

Without reading the manual, I connected my portable MP3 player's headphone output to the RCA jacks on the rear of the unit (assuming that they were AUX input jacks, not subwoofer out jacks as they actually are).

Here's the kicker. It works fine. I can hear the output of the MP3 player (albeit only through the rear speakers). I'm only using the MP3 player for audiobooks, so sound quality for it isn't really an concern. My real question is this: what is the chance that I may somehow damage my head unit by doing this?

Any (non-flame) replys are welcome. Thanks.