Hi it does sound like wiring I have a 2004 c3 and that ended up being a corroded earth in the rear light connector might be worth pulling it off and checking the connector see if it looks damaged at all
Hi just joined the forum looking to be of help to anyone that needs it and also advice if I need it :) I'm a citroen consultant technician and fellow citroen owner (2004 c3 1.4 petrol).