Car Alarms: 2000 Mustang Alarm, keyless entry system, starter wire

Question
I have a 2000 mustang GT that I purchased used.  My wife was driving the car when she stopped and got out of the car to pick my son up from the babysitters the alram armed and now the car will not start.  The alarm starting going off and the keyless entry system will not unarm the alarm.  We tried changing the batterys on the remote and still won't work.  The alarm has shut down the entire electrical system and now the car won't start or even lock the doors.  I have looked under the dash and do not see a kill switch.  The only thing that i see is a red flashing light on the dash.  You can push the light but I have no idea what it does or even how many times to push it or anything else.  I have no idea what brand the alarm is since they tied it into the factory keyless entry.  Any help would be appreiciated.

Answer
Without knowing the exact make and model of the system I can offer general suggestions.

Most systems that are tied into the factory keyless entry arm and disarm by monitoring the door lock motor wires.  When it sees a positive pulse on the lock leg it arms when it sees a positive pulse on the unlock leg it disarms.  If unlocking the door did not disarm the alarm there should be some type of manual override.  The manual override is typically achieved by turning the key to the run position and pressing/flipping a button/switch.  Some systems incorporate the LED and Valet switch in one device (as you have explained your LED pushes in).  If you can not disarm the system by turning the key to the run position and pressing that button I suggest disabling the system by removing the dash panel under the steering column, tracing back the wires that were tapped in and removing the fuses that supply the aftermarket piece until you can get a chance to have the system looked at.

When you say the alarm has shut down the entire electrical system, this is not really a function of the alarm.  The alarm will have a relay tied into the vehicle starter wire that prevents the starter from working when the alarm is activated.  It does not have the capability of 'shutting down' the vehicle's electrical system.  With that said however it is very possible that the system caused the vehicle's Body Control Module to lock up.  This should be easy to unlock by simply disconnecting the vehicle battery for a few minutes and reconnecting it.