as far as i am aware the module under the seat controls both front end rear parking sensors, there is a buzzer next to the module under the seat this one sounds when the front sensors are detecting something and the car is moving forward and the rear buzzer sounds when the rear sensors are detecting something and the car is in reverse. i have had the module apart before and as far as i am aware there are no screws to remove you just need a small flat bladed screwdriver and carefully prise the front of the box off. Theres not a great deal inside but it won't do any harm to give it a good dry out and blow out. You can also check the sensors out, put the ignition on but dont start the engine and put the car into reverse so that the sensors should be activated go round each sensor in turn and listen for a very faint ticking noise from each sensor. if none of the sensors tick, front or rear, i would be guessing that the module is a fault if all of the sensors except for one tick then its probably that sensor at fault. if this is the case take one of the good (ticking) sensors and swop it with the faulty one and see if it still ticks, this will determine if it is the sensor or the wiring to the sensor. You can always take it to and auto elec and get them to plug it in and they should be able to tell you exactly whats at fault.