I've found the Peripherals adapter that lets you plug in a normal head unit into the amplifier, but if I'm right, that just converts speaker level outputs down to a line level for the amplifier.

If it's the infinity amplifier, it needs a level somewhere below speaker-level, and somewhere above line-level. Any adapter you get would be a level-matching transformer, and it would either have to amplify up or attenuate down, depending on the input level.

I did an install to an Infinity system, and with the right adapter, it was very "plug and play", so I'd consider this a good thing that you've got the infinity system. Details of the install are here including screen shots of all the adapters used and their part number tags.