GM-GMC: 2004 GMC Sierra 2500 HD Pickup 4-door, gmc sierra, cold weather

Question
My GMC is acting like it is possessed.  It has been at GMC for several days and they can't figure it out. It won't start in the cold weather at all but if you jump it then it will go.  If you leave it in the garage it starts every time.  Today they say the after market security system is wired directly into the BCM and they want $250.00 to uninstall it to see if that is the problem.  (shorting out the BCM).  They said if that doesn't fix the problem then they will put in a new BCM.  What do you think?  Why would it only happen when it sets out overnight?  This doesn't make sense to me.  Please answer.  Thanks

Answer
It is entirely possible that your A/M security system is doing funny things.  Why it only does it after sitting outside?  I couldn't even guess.  I am interested in knowing what is preventing the truck from starting.  I would approach it like any no start condition.  Is there fuel pressure, if yes, is it the proper pressure, if no, why not?  Is the pump getting power and ground?   Is there spark?  Is there any diagnostic trouble codes (ie theft or security)
Stuff like that.  Any good technician would have covered all of those bases before looking at BCM's or security systems.  How is the fact that the truck starts when boosted figure into the BCM diagnosis? That being said I would agree however that in order to diagnose any system the aftermarket goodies should be removed.  Is $250 a reasonable price to remove it?  I wouldn't think it should take no more than an hour and half or two hours at the very most.  Depending on what the labour rate is at the dealer, you may be in that range or slightly lower.  
  One question that should be asked is who is paying the bill if after the security system is removed and replacing the BCM doesn't fix it?  I wouldn't be paying a cent if it were my truck.  
The only advice I can really give is to ask questions and document what they are doing and what reasons they have to believe that is the case.  Far too often customers are stuck with bills for blind guessing rather than logical diagnosis.
Keep me posted.
Hope this helps.