+1
If you have aftermarket alarms or devices installed as generally are on JDM's your at the mercy of the installer- depends how much care they take - if its a get it in quick and dirty job as he has 5 to do that day and will get his £300 per car then he wont care, a lot of work I've seen like this has been messy - messy wiring never trimmed to length just rolled up and tucked out of sight! Half the time these cables get interference or cause interference - somehow I knew that the alarm company would deny any issues before I even got to read that part of your post!
A good auto electrician will be able to diagnose it for you hope you get it sorted soon