about the keys...the installer should have given you these when he sold you the stereo. i would go back and get those. tell him you dropped m&ms back there and need to clean it out. they keys come with the stereo, so you own them.
as for the factory amp / aftermarket amp / head unit / harness fiasco....
i agree; since you have a new head unit and a new amp, i think you need 5514. what that should do is connect the power to the new head unit with one harness, connect the speaker wires to the speaker harness which runs to the amp, and then use the eliminator harness to bypass the amp. a line level converter should not be necessary since you're not feeding any signal to the factory amp, you're bypassing it! right?
then, the line-outs from your head unit should run directly to your line-ins on the sony amp in the trunk. the sony amp should have a dial which can be used to set the sensitivity for the line-in so you get a good match between the amp and the head unit.
