If you are experiencing problems with electric windows, mirrors, speaker, central locking etc., the problem may be due to fractured wires in the door loom. This happens due to the cables flexing slightly each time the door is opened and closed, and 25 years later they break. Its usually the drivers side.
Repairing is generally fairly straightforward. Its usually the drivers side as this door usually gets opened more often.
Firstly. Remove the drivers sill trim. This covers the footwell fusebox. Its mostly plastic peg connectors, but there is plastic nut in the footwell to undo. This is usually finger tight.
The door loom plugs into the bottom of the fuse box assembly. Fuse box is retained to the body with a screw, and some clips. With this loose you should be able to unplug the door loom. If you tie a length of string to the end, you will find it slightly easier to pull it back into the car.
The loom exits the body in the door jamb and has a rubber cover which can be gently pulled away, showing a white plastic clip. A small screwdriver should release the retaining tag and allow the loom to be extracted through the body. You can also unclip the wiring clips which hold the loom to the door.
Sliding the rubber sheaths back should reveal the wiring and it should be apparent where the broken wires are. You can then cut out sections of wires and introduce new wiring to replace sections. Try to use wiring of the same cross section diameter, and if you have more than one wires to repair, if you stagger the joins this means the bundle of wires doesn't swell in one place with lots of extra connectors. How you repair is up to you, but I usually use butt connectors with heat shrink sheaths - make a good crimp and then heat the connector with a hot air gun or cigarette lighter to activate the heat shrink which helps strengthen the connection. I found whilst doing my car recently that someone had repaired two wires previously, but just twisted wires together and taped up the joint. Whilst the repairs hadn't broken at the joints, it wasn't a neat solution.
If you have lots of broken wires, there is the argument of rebuilding from scratch, in which case its probably easier to extract the whole loom from the door, and then you can reconstruct it on the bench, but you would need to spend time extracting the wiring from the terminal blocks and sourcing replacement connector terminals of the correct type.
If you have a spares car or would prefer to build a replacement before you take the car to bits and want to copy something, please note that phase I and phase II cars use the same door looms (03/90 to 06/92), but phase III cars from 06/92 onwards use a different loom. The RH and LH door looms are also different, the drivers door has more wiring - the passenger mirror control, and the passenger side window switch