Questin - My car often cuts out during braking and if it doesn't it almost always affects the engine and power, for example the interior fan will lose speed and power and anything lit will dim. What are the possibilities and could I check/repair this myself? How do I narrow down the fault?
Answer - Hi Mark, The explanation is a bit misty, but I will give it a shot.  When you press the brakes, there is a switch above the breaking pedal that activates the breaking lights. The switch closes the electric circuit. What you are experiencing is a power failure due to

1. A short circuit somewhere in the braking lights installation. Look for cables that are cut or the isolation is damaged. In this case the fuse should have blown out. Check that the right fuse (regarding amperage) is in place.
2.  This is most likely. When the installation does not have proper ground contact (thats the wires that are connected to the body), odd things like this happen.

Check the wiring from the rear lamps for cuts or damages on the installation. Also open the lamps and check the bulbs and contacts. Bulbs are known (rarely) to make short circuits. Look at the wiring that goes from the back lamps to the front of the car. You will find a cluster of wires that are connected to the body. Check them for corrosion or loose contact. Also, look at the cables that go straight from the battery to the body of the car and the thick cable that goes from the body to the engine-transmission. This is subdued to corrosion and can make a lot of problems.

One more thing: if you have a multimeter, check the voltage input, just in case. It should be from 13.8 V to 14.5 V Hope this is not too much, but this is something you can do on your own. If this does not help, write again and we will try something else. This time I will be more expedition.
Have a nice drive,
CRG team