What would keep making battery die

We have problems with the van, the battery keeps dying. We've had 3 brand new Interstate Batteries put in since May 2010.
Our mechanic cannot find any problems, other than battery issues.
The battery is good then 5 to 6 months later, click click sound.
Have to jump start it and it's fine for awhile, then same issues.
Advance Auto tested battery and alternator. Tested fine.
We do have alarm on car and now the automatic door locks do not work.
When you drive, over 18 mph they are suppose to lock automatically.
We have 2 sliding doors, neither lock with the auto key, you have to manually lock and un lock them.
The guy from Advance, thinks it could be a problem with a relay.
Can you help? Tired of being stranded.