Retrofitting a tachometer
You can always have an aftermarket tachometer sold in many car parts stores. However, these are additional units you have to place somewhere on the dashboard; not a nice thing to do if you want to keep the looks of your car as original as you can. But if you replace your instrument panel unit with one having a tachometer, you can have the same layout BXs with higher trim levels originally had. With a little work you can change the tachometer only but it would be even wiser to look for a complete GTi instrument panel and retrofit the coolant temperature gauge, too, which is also lacking from your dashboard. Study the description of this modification first.
You can connect the tachometer using the 4-pin connector at the bottom of the unit. Pin 1 (arrowed on the picture) is the feed switched by the ignition key (in the original layout this wire forks from the one coming from the mainboard V/5 (yellow 4-pin) connector and goes to the oil level gauge as well). Pin 3 connects to the low voltage connector of the ignition coil (the one connecting pin 1 of the ignition module to the coil). Pin 2 goes simply to the ground. Pin 4 is not used. You can cross the bulkhead wall with the necessary wiring beneath the battery.
You will also have to think about the redline zone on the tachometer. If the meter you have just installed comes from a BX with a different engine, the redline zone will be different. It is highly recommended to modify this marking to reflect the behavior of your engine. You might not forget it but some nitwit at a garage might not pay attention and torture the engine into too high a revolution during some tests or repairs. You can make red marks with red touch-up body paint. First, cut out the negative image of the required shape from Scotch Magic Tape or similar and use this as a stencil pattern to give it a nice look. Some paints may attack the plastic of the front panel or might not leave a nice clean edge when you peel off the adhesive tape used for a pattern. You might want to make a first try on the invisible edge of the front panel, covered by the facia frame.
As a second option, you might also buy a red self adhevise label or print a shape onto a transparency film with an inkjet printer and glue it onto the front panel--this second solution is easier to rectify if it does not look nice for the first try.
In theory, you can do the same in a diesel BX, too, the only difference is that the tachometer has more connections and an additional sensor is also required as there is no ignition signal to feed a tachometer in a diesel engine. As soon as we locate the necessary wiring diagrams, we will describe the modification for diesels, too.