Keyless Entry problem

I have a '95 ti which I love. But a problem with the alarm has developed.
I push the top button and the doors lock. I push the middle button and the
alarm is armed but 5 seconds later it will go off. I then have to disarm by pushing the top button.
The alarm light on the dash will blink twice for about a minute indicating the trunk sensor was activated.
It seems this problem started after I replaced the battery.
Any ideas?
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