some stuff 'talks' between body control and engine control ecus depending on what systems are used, like if the body control units works the fuel pump, it will take a signal from the engine control and other things like crash control ecus etc.. beleive me, when I worked for the AA I came across some of the most bizarre setups that have you standing at the side of the road scratching your head!!!! you'd be amazed what can be acheived with a fused length of wire!