99% of the time the aftermarket door lock actuator (that they fit with the alarm) shits itself over time and gets weaker... have the same problem right now on my EG but im too lazy to fix it..works on and off

The actuator is fitted inside the drivers door and looks like this:

http://www.auto-accessories.com/powerdoor/80174big.jpg

You can either buy it and replace it yourself or take it to an auto alec/alarm installer...relatively easy fix