My 2003 mitsubishi eclipse spyder anti-theft system is on all the time. Every time I open the door the headlights flash and the horn sounds (very loud). I tried to disarm it according to the manual but nothing happens. I took the horn fuse out and it just changed the tone of the horn and kept on honking and the lights keep flashing. It continues for about 3 minutes then stops, but as soon as I open the door it starts up again. Any suggestions?
I've managed to disconnect the other noise-making thing (Horn?) so, at least it's just the flashing headlights now, but something's not right??

That is definitely an alarm issue.
You need to get this in the hands of someone with the appropriate manual to test that alarm.