Been away a while

What I'm not having fun with is a recent alarm install, it's a Cobra with dual immobiliser, sonic sensors, yada yada. The issue is that it will never unlock, only lock (but it will always disarm, it just won't trigger the door unlock).
The car has a Toyota keyless entry thing, with a fancy key with a single button on it that triggers both locking and unlocking. The Cobra remote has a single lock/unlock button as well.
When the Car Stereo Company installed it, they said it was "intermittently" not working and a Toyota fault. This is quite clearly not true as it never works, and the Toyota key still works fine (although obviously won't disarm the alarm).
Why I'm posting is that the car's going back to be looked at by them tomorrow. I'd like some thoughts on what you think it might be, be it a fault in the internal system or where the hookup with the Cobra alarm might be at fault. The engineer in me thinks that it's unlikely that there's a fault in the car itself since the stock key works fine, and that they should just be using the same system to make it work.
Thoughts?