You will have extra factory wires, connect that ground to a bolt or something under the dash. Blue goes to your power antena if you got one. You should have a white w/blue stripe and a solid black wire coming from the factory harness, it may be cut up high so you don't see them. Double check the colors of all the wires you already spliced too.

http://www.installdr.com/Harnesses/Mitsubishi-Wiring.pdf

*Edit* Just looked at my harness and the grey w/black stripe wire connects to a black w/red stripe wire. White to white w/blue stripe.