If you came from other Citroëns, most notably CX or DS, you might be disappointed. Citroën—to save room in the engine compartment but also for economical reasons, to share some components with Peugeots—redesigned the front suspension. The traditional layout had a triangle trailing arm (the suspension cylinders and the wheels attached at opposite corners of the triangle) while the new setup uses McPherson struts (the cylinders and the wheels attached at the same point).
Any work on the hydraulic parts below the car should be carried out with the car safely supported on axle stands or standing over an inspection pit. Never work or allow anybody to work on the car (not even for a small adjustment or even just inspection) when it's only the high setting of the hydraulic system that keeps the car up.
The speedometer cable is split into two parts but that doesn't mean you should replace only one part, unless you are positive that only either the upper or the lower part is failing. But even then, the other part may follow its counterpart in a short notice...
Although synthetic oils are perfect for the engine, don't use them in the gearbox. Citroën gearboxes (the synchro rings, actually) seem to rely on mineral oil which is thicker than its synthetic counterpart. I committed the same mistake: trying to be smart and go for the best, I chose Mobil Mobilube 1 synthetic. Although it worked, changing gears became harder. A couple of days later I went back to the originally specified oil (Total BV 75 or EP 80 are mentioned in my instruction manual, Castrol also manufactures the second) and it's as smooth as you can get.
On Series I it is located where the AC pipes go through the bulkhead (at least for a LHD car, I have no idea on RHD). The bulkhead bridge is an oval metal part surrounded by rubber, with two pipes leaving it. It is bolted onto a similar part on the inside of the cabin which is a part of the heat exchanger, literally through the expansion valve assembly. What you do is (of course once that the remaining gas has been evacuated) to unscrew the two nuts that hold the oval part on the engine side of the bulkhead.
If the carpet is constantly wet in the footwell and you lose coolant slowly, no doubt, the heater matrix (the actual heating element) is leaking. There are two pipes crossing the bulkhead behind the engine, on the right side. Look up beneath the lower trim panels in the cabin: the pipes go through a black box, the valve you open and close by turning the red-blue control knob on the center console. If the valve is opened, the hot coolant flows through the heating element, transferring its heat to the air blown through it.
The theoretical solution is plain and simple: lubricate it. In practice, however, this means removing and disassembling the motor (as described in the answer to the previous question)—not something you would want to do every six months or so, wouldn't you? But if you do it once, you can modify it so that you can lubricate it later as often as you feel necessary without going through the hassle of removing it any more.
If it's not working at all, start with checking the fuse #2. Remove the lower shroud of the dashboard (secured by seven screws). A relay is fixed to the right side of the steering column. Turn on the ignition. As you turn the blower speed knob to the full speed position, the relay should click. If it doesn't, check its feed (connector #1). Then jumper its connector #2 to the ground, this time it must click. If it doesn't, it's faulty. If it clicked this time but it didn't when you have just turned the knob to the full speed position, you have to check the heater controls.
This is rather easy on the Series II models (but read on, we have some ideas for Series I, too). The modification has been done on various models already: 15 TGE, 19 TRI, 19 TRS, 17 TZD turbo, 19 RD, 19 TZD. As far as I know, the exact same procedure should apply to all BX 15/16s (but it would be nice if someone could try and inform me about the results). Part of the modification might work on a BX 14, too (temperature meter but no warning lamp).
Avoid products which claim to block any leak from the inside (Holts Radweld and similar). They can clog more than the leak itself, the heater matrix or the smaller passages inside the engine might suffer. Do it properly and thoroughly instead.
First of all, locate the leak. Look for the usual whitish residue around hoses and clips. If a hose is ruptured, replace it (on the roadside, you can use Holts Hoseweld to bandage a hose but replace it with a new one as soon as you can). If the heater matrix is leaking, replace or repair it.