The XM is a complicated beast when it comes to electrics and electronics: there is no denying that. It entered the executive car market with practically all the toys of the era. There are billions of components, thick harnesses, a plethora of connectors, everywhere. Finding anything without the proper diagrams is nigh impossible.
There are two parts that might need replacing in the blower motor: the control module and the brushes.
You might suspect the control module when the blower doesn't want to run at all (although there might be other reasons for this as well). Brushes are likely to be worn if the motor doesn't start one day but runs fine on the other. In this case, a moderate whack on the motor from below starts it nicely.
The XM Series 1 was born in the era of automotive bulbs. When it came to the Series 2, LEDs were already entering the scene. New switches like those on the dashboard started to come with LED illumination but earlier switches retained (and shared with other Citroëns of the time) remained with plain old bulbs in many switches of the door windows, electric seats or some center console switches. Retrofitting LEDs to these is a simple process with huge benefits: longevity (practically, eternity) of the LED, cool operation temperatures and lower power consumption are all welcome changes.
The jockey arm type auxiliary belt tensioner is no longer available from Citroën. If yours starts to rattle, warning of an impending failure, you only have the option to modify it. There are two possible ways to do it: either fix it permanently or use a replacement tensioner with the necessary modifications to fit it.
Troubleshooting the electric circuitry is often feared but is very straightforward in reality.
Start with a copy or printout with the circuit diagram in question. We take an example here, the driver's side electric window of a Series 1 XM. A printout or a copy will enable you to take extensive notes on the diagram. Create as many as there are actual operating states of the circuit. In the case of the electric window, this might be:
This number shows the day when your BX was actually assembled on the production line. The dealers and parts stores use this number (often called ORGA or RP number, this latter stands for Replacement Parts) to identify the various parts and components fitted to your car. You'll find this number either in the engine bay, in front of the left front suspension sphere or on the A-pillar, in the door opening, below the hinge.
First, congratulations! You'll soon find out that there are only two kinds of car drivers: those who drive Citroëns and those nincompoops who don't... :-)
Start with a major cleanup of all compartments: clean the carpets, under the front and rear seats, in the trunk. Clean the exterior, the wheels. Check the car body thoroughly for scratches, small dents and rust. Rust should not be a problem on a BX unless it was neglected or some repairs were carried out without paying attention to prevent rust formation. Repair as necessary.
Discolored or broken vinyl trim parts (even if some morsels are missing between the broken parts) can be perfectly repaired using Araldite (or similar) two component epoxy adhesive resins (choose the fast-setting version). The customary plastic packaging used in great volumes today (with paper background and the product shrink wrapped) can be used in the process. Cut out the bigger flat parts of such plastic wrappers. Araldite does not glue to them: even if it seems that it does, it can be easily snapped off once the resin is completely hardened.
Avoid silicone spray. Many trim areas—especially parts of the dashboard exposed to direct sunlight—are constantly bombarded by ultraviolet rays which break down the molecules comprising the vynil skin, allowing raw vynil molecules to escape the panel.
Do it yourself! Touch-up brushes are unusable but Holts (England), Multona (Italy) or Dupli-Color (Germany) sell the same acrylic paint in 400 ml spray cans, and those are excellent. You can find the matching paint in their catalog based on your car's paint reference number. You may have to check more than one of those, their color range is not exactly the same (for instance, I could find the exact match for my Red of the Long Valley only in Dupli-Color and the Anthracite Grey used on the upper frame of the doors only from Multona).