A kill switch is recommended to be installed close to the power source, in this case the battery. Remember that e30’s battery cables run from the back of the car to the block in front passenger side, which in case of an accident will remain powered for that distance (almost 7 ft. or more).
Also is important to consider the most basic principle that while racing, cars run in forward motion which increases the probability of a frontal accident. People that located the switch behind the hood, there is the possibility that in a frontal accident the hood moves over the device making it non functional, or covering that area. For these reasons I think the best place is in the license plate area.