Factory Alarm on Ford Explorer

my 1999 ford explorer eddie bower editon has developed alarm troubles. each time I lock the car, the alarm goes off, the lights blink and I can’t stop it. the alarm goes off and stops intermitently. I can not leave the car unlocked for fear that it would be stolen. Help; should I go to an alarm place and replace the alarm?

The problem may be due to one of the door switches not opening like it should be when the door is closed so replacing the alarm may not help. Fords have problem with the door switches doing that but cleaning the contacts in them clears the problem. The interior lights should be on also if a door switch is having a problem.