bmw e36 1998 328i Harmon Kardon Amp question

I upgraded my Audio amplifier to a Harmon Kardon. The HK amp has 3 input ports. Two large cable trunk inputs and one small one. What is the small input used for? It appears to have only two wires feeding this small input port (from pictures I've seen).

thanks guys !!!

Not really sure w/o seeing pictures. In general, the inputs to an amplifier are:
1) power (usually thickest wire)
2) either RCAs or speaker-level inputs
3) signal turn-on (skinny wire, tells the amp to turn on when the head unit is turned on, and off when the head unit is turned off)

outputs:
1) ground (also thick, usually same thickness as input power)
2) wire to speakers
3) other - if you are powering another amp, this amp may have outputs to power/feed audio to that other amp

good luck!

Go to the "Useful Links" sticky and click on the 2nd link. It will get you to a PDF copy of the Bentley manual which has full wiring diagrams for the E36, including the audio system.