Unfortunately, the noise is inherent in the system: the lock motors are noisy themselves (not the motor itself but rather a disengaging mechanism inside), the rods and latches attached add to this, and the whole is vastly increased by the hollow door construction.
The remote control on a BX uses a TEA5500 chip for both the transmitter and receiver. This device has ten programming inputs and each input has three possible states, connected to the high or low bus or, unusually, left unconnected. This gives 59,047 possible combinations of code.
You can spare the expenses by modifying the new switch to match your original key.
The sunroof is not intended to be watertight, the water that gets through is drained by four drain pipes. The front ones are behind the plastic trim on the A pillars, the rear ones behind the C pillar trim and they usually get blocked. Never try to clean them by rodding or blowing into them, they can puncture or separate at the joints. Suck the blockage out of the pipes using a vacuum cleaner instead.
Generally, you could find out where the water is coming from judging by the traces of water or by removing the footwell carpet and asking someone to pour buckets of water onto the car. If you have a sunroof fitted, this is the first place to check. There are water drain channels in the C pillars, if one or both of them become clogged, water accumulates above the headlining and comes down generally behind the B pillar trim.
I had nothing but bad experience with Bosch wiper blades, be it revolutionary, twin, aerodynamic, whatever. I stick with Champion and Valeo. But one word of caution: when it starts to streak, never rush to buy a new one. Get some alcohol and kitchen paper towel (it's better than Kleenex). Pour some alcohol on the towel, pinch it between your fingernails and literally scrape the blade edge, but hard. Dirt can stick to it with great force. You do this twice or thrice and lo and behold, you blade will probably return to its new condition immediately.
The mainboard is located under the lower trim panel on the left side (both left- and right-hand drive models). Pull (Series I) or twist (Series II) the release lever and swing the box down.
When it is not raining too hard, the water wiped to the left side of the window, comes back up because of the wind and hinders the view through the windshield. Other cars having only one wiper arm—namely Mercedes—have the resting position on the left side of the windshield. This can be achieved on the BX easily: remove the wiper arm and the plastic cover above the wiper and blower motors. Rotate the small iron arm of the wiper rod mechanism by 180° so that the spindle of the arm would rotate in the opposite position.