I have a 2003 BMW 1200LT that I just bought used. Never had any alarm go off in the last few months that I have used it. The other day my son left the switch on in the parking light position and the battery went dead. I raised the seat and hooked up a low amp/slow charge charger directly to the battery posts. Now, after charging, when I turn the switch on the anti theft alarm and immobilizer are activated. In other words, an alarm sounds for a few seconds, lights flash, and it will not crank (starter will not turn over). One thing I noticed is the light just below the headlight adjuster knob inside the cowling is blinking. It never blinked before. I do have the remote control key fab but do not know how to use it. I tried pushing the buttons and some LED lights flashed but the alarm continued. Someone said there is a secret code but I do not have it ad can't get it from the previous owner. What recourse do I have? Thanks

Optional Information:
Year: 2003
Make: BMW
Model: 1200LT

Already Tried:
pushing buttons on the remote key fab... turning the switch off and on.....removing and reinserting the key.