I didnt know we had a ground in the factory harness. I thought mitsubishi you have to ground it to the car itself. I dont know about the blue wire. Usually the blue is either antenna or amp turn on. If its amp turn on, and you dont have an amp you dont need to connect it. Since the deck is used, I would take it to a place that sells car stereo stuff and have them test the deck. There are shadey people on ebay that will sell anything, especially a broken deck.
-Todd

Star Stage II, Greddy Type S, ADFX 3" DP, blah blah blah