When I shunt a car and lose the rear wing and rear bumper, I enter the pits and select 'fix rear aerodynamics'. However it only fixes the rear wing and it will never fix the rear bumper.
Same problem arrived yesterday for a friend with his SR3.
Stop to pit for reaparing the rear wing but not seems to be done.
His car was like a soap on the tracks after the pits with no possibility to terminate the race.