With Citroens up to the BX (and possibly beyond) one of the simplest tasks is to remove the tail lights.
In the case of the CX & BX they are held in place with a piece of spring stainless steel that is tapered in a wedge shape. To remove, it's a case of pressing this down until the entire assembly pops out of the hole it sits in. The bulbs are on a small PCB which is usually held in with a simple 'slot & pegs' type of arrangement at which stage you are left, after about a minutes work, with the bare lens assembly.
Firstly, wipe the bulbs over to clean all the dust off and then remove the bulbs one at a time and clean the base and connecting points. Check to see all are working & replace any blown bulbs or tighten loose connections.
Take the lens assembly and put a few drops of detergent inside and pour water in. Leave soak for a couple of minutes. Then agitate by shaking vigorously. A small clean paint brush can be used to fully clean internal corrugated surfaces, detergent water poured out and fresh water flushed through. Shake as much water from within as possible and ideally blow out until dry with compressed air. If this is unavailable, leave in a draining position in the sun for a few minutes until dry & refit to the car; you'll be amazed at the difference to the lighting that is made.
I have to live around dirt roads so I have to do this more than most, however anyone living totally on sealed roads and particularly in large Cities will be surprised at how much grime has accumulated over time in those nooks & crannies.
A further word of advice; also be sure to do the covers on the number plate lights as they are clear and will look quite daggy against your newly washed tail lights. The amount of grit on these is quite easy to see when the boot is open & is a good guide to how bad the tail lights will be inside.
Alan S
Cleaning tail light lenses
Moderator: RichardW