On the dash is there a switch to disable the reversing sensors that might have been touched or maybe something in the radio Menu ? The chime when putting in to reverse is part of the reversing sensors so once the sensors are working this should work. Something else to check, are the reversing lights working ?
This topic has a procedure for removing the glove box and mentions the wiring connectors that are moved http://www.citroen-o...love#entry95604
Another possibility is disconnecting the battery too soon after removing the key and not allowing the bsi to shut down. I have used the following from the Common Problems topic when disconnecting/reconnecting the battery:
16. Odd electrical problems caused by wrong battery disconnection procedure.
If you need to disconnect the battery and not do a BSI reset procedure, I have found the following has not given me any problems on a 2004 and 2007 C5 and a 2003 C3:
- remove the ignition key from the car
- open the bonnet
- wait a few minutes for the BSI to shut down
- disconnect the battery negative terminal
- ignition key out of the car
- reconnect the battery
- wait a few minutes
- get in the car and close the door
- insert the key in the ignition and wait a minute
- turn the key to the first position and wait a minute
- turn the key so the dash lights come on and wait a minute
- start the engine
- take the windows down and up to reset the anti pinch
- reset the clock (and radio on the C3).