Stereo in my 91, Ipod?

[hiftbso91]

I have a 91 with the Ford prem. sound system, not JBL, and I want to find a way to hack into the factory amps with my Ipod and bypass the head unit. Does anyone know what kind of inputs the factory amp(s) have and/or if you can send signal straight to the amp with a 1/8 inch plug or RCAs. Any info on the sound system would be great; amp ratings, speaker sizes, ect...
I am new to this forum and the SHO so help me if you can.
Thanks, Mark