well like said idk if that plug is to hook the sub and amp to or not.
Someone correct me if I'm wrong but I'm pretty sure that's the amp plug
Yes it is. My old 2000 also has the plug. IIRC, all LSes have it regardless of audio setup.
when i installed my replacement 6 disc head unit there were no empty spots to plug into left over so i guess that everthing that could or would get pluged into my head unit is pluged in. can i test the plug in the trunk with a multi meter to see if its getting a signal from the head unit?