The only way is to pop the lock, but it has a tamper-proof device to where it can't be fixed at that point. The only option is to replace it with a new one, but be VERY careful or you could crack the glass. This is messed up on mine too, but I'm letting a mechanic do it since I don't want to replace the rear window. If you do it yourself, be sure to have a new lock assembly for the back on hand and be careful with the glass (my rear glass won't open, maybe the button still works on yours - in that case, open it first).
Horn is either to the front right or front left, I can't remember off hand. As for the wires, I'm not sure, but check the fuse first.