Does anyone have any idea how I can stop my SWAT team jumping out of the bearcat every time i stop at a red light

I think it's hardcoded into that model that they will fan out as soon as the car starts. 
You could try using the noose helicopter pilot model instead. 

In truth, any model that is assigned to the COP ped group will keep getting in and out of any police vehicle you will try to make them get in except when the vehicle is moving. It is indeed hard-coded into the game and there's nothing you can do to change this behavior.

 

On a side note, the NOoSE helicopter pilots are not assigned to that group, so ineseri's suggestion will work just fine. You'll just have to replace their pilot helmet with the ones used by the NOoSE officers.