Citroen C5 reverse lights flash constantly

Check reverse light switch on top of gearbox and wiring

sounds like reverse light switch but strange it happened after fitting new battery you could try a re-boot set suspension to lowest position disconnect both battery leads and leave for at least one hour then re-connect but dont operate anything for half an hour then see if problem has gone worth a try

we also had a new stereo fitted a few months ago and had to turn headlights off of auto has they stayed , then other week the battery died so had a to fit a new one then the central locking would not work , so had new batterys in keys , and reset them to the car , has shown in a video , and they worked , then we had the aircon filled , after a week it works when it wants to , then the sound started playing up on the stereo , then the off side rear door wont ulock with the central lockin you have to open the drivers door and lock the car and the door will open , then yesterday the reverse light was flashing constantly when the locked and on full when driving ..........

starting to drive us insane now tried bsi reset still doing same just trying the leaving the battery off for a hour now , will let you how it goes later ............

Nope that didnt work looks like got to take to a specialist if they can get me b4 we ment to go in a week .... :(

Ask your local specialist, not dealer, to clear all fault codes, sometimes the system retains access and behaves irrationally. When fitting new electricals the system has to be informed.