Do you have a multimeter? If so, set it to the 200 Ohm range and put the probes on the left two legs of the fire button, if it reads 0 when you press the button those are the switch connections and you can solder to anywhere on the tracks they connect directly to, just scrape off the varnish in a clear spot and solder the wire to that. Failing that you can carefully solder to the actual switch legs, you will only need fine wire for this as the switches are basically used as logic signals, they carry very little current (Closing the switch usually raises the input on a chip from 0v to +v) it does not supply the actual power to the atty unlike a mechanical mod...