You want to change front bumper to a heavier style do you need to worry about air bag sensors?
The air bag sensors are on the frame so they will not be a problem. Mark
It is behind the front bumper, and there are two of them, one on the left side and one on the right side. It is easily accessible if you lift your gran caravan and look under both corners of the front bumper. The sensors are attached to the chassis with two screws. Just remember to unplug the negative cable of the battery before doing any work on these sensors.