The Black Wire on the left is the ground wire, if the conector was cut off it should be the shorter of the two black wires of the wires that come out midstream of the wire harness.
You can take a test light and find a known power wire and strip the end.
Now take the grounding clip and clip it on the bare power wire, take the tip and touch the black wires to find your ground.
Be carefull what you touch when you do this, you can blow a fuse or shock yourself if you are not carefull. Remember that the power wire and clip can short out as well as the tip.....
If this dose not help you, find a friend that has worked with car audio or security systems to help you, or take it to a shop and have them do it for you.
Not to be mean but if you have not done this kind of stuff before it is not a good idea to start cutting wires and tearing things up with out knowing what you are doing. Allways mark the wires before cutting them and know where they go to. If you want to learn how to do it then get a friend to help and teach you. You can realy f%$# things up if you are not carefull.