Ted-Michael Sanford

ok so when my dad got his mustang it didnt come with a radio, i do not know whether it has premium sound, mach, or ...regular. in the dash there are 2 sets of connectors bundled together. on one it has a rectangular plug and a square one, on the other, they're both rectangular. when i try to hook my radio up to the set with the square plug nothing happens, when i hook it up to the other the reset light on my radio blinks, when i hit reset.. it continues to blink. im completely at a loss and very frustrated. any help?