Ford Mondeo i have to hit the black box under the back seats for it to kick over

any impact causes vibration which can make a contact occur where before there was none. the mondeo system is very complex and sensitive, parts like computer boxes are easily damaged by intermittent voltages or "spikes" created in this way. disconnect your battery, unplug the unit you've been hitting, check for corrosion or dirt, spray in wd40 and work the plug on and off a few times (good for towbar electrics too)reconnect battery. failing this check wires for stiffness at the plug, a loose one may just be hanging by the insulation. check the loom for damage and potential shorts. last ditch, take apart the black box and look for broken tracks or cracks on the board and solder them up.