2005 tacoma front door lock problems

I installed a Viper 791xv in a buddy's 4 door 2005 Toyota Tacoma in June of 2006.  The factory door locks require the double pulse to unlock (where the factory keyless entry system required to hit the button once for the drivers door, then again for the other 3 doors), and at the original time I installed the alarm I programmed it to do just that.  It worked fine for a number of months.  Recently, he has let me know that when he arms the truck, intermittently (though more often than not) the driver's door will not lock, and in about half of those instances the front passenger door will not lock either.  The rear doors always lock.  He also reports that this will take place at times when the lock button is pressed on the door (so not just when using the alarm).

First question (and the one I'm really hoping for); is there a quick easy fix that I just don't know about?

Second, where is the best place to start troubleshooting?

Third, is this something that is more likely on the vehicle side?  Or maybe a problem where the alarm integration has deteriorated the factory system?  Or just something on the alarm side (which I don't think is a reality, seeing as I only tapped into one wire for lock, and one for unlock)?

Anyways, I'd love to help him out with this, and I'm a little far removed from my days as an installer so any assistance is much appreciated!

(On another note, it's scares me to realize that I've not worked as an installer for 4 years already!)

For what I have in tech support the info you posted (draasch) is for a 4runner, not a Tacoma, they are different.

What I have for the Tacoma is

Lock and arm Purple, in the drivers kick. White connector that appears to come thru the door.

Disarm and unlock (double pulse) GREEN/ black, same connector.

I don't have any idea why I am posting this, because CRVZ said it worked fine, then quit working.......?