The EPC light is a bit of an oddity, on some VAG cars it is the Engine Management warning light (i.e. it lights up when the ECU logs an error, which you then read on a scan tool). On others it is not the ECU light. For example I have an Audi A6, that has an EPC light and another light which is an engine with a spark in the middle. That is the ECU warning light. So what is the EPC light? EPC is Engine Performance Control, or Engine Power Control, most VAG cars now have drive by wire electronic throttle controls, so no throttle cable. The ECU use a thottle position sensor to operate a motorised throttle body. The ECU monitors a number of systems in relation to the control of the thottle. One of the systems is the brake circuit, i.e. it checks the see if the brake lights are working. If it detects they are not it lights the EPC light and may even go into 'limp-home' mode. You brake lights are not working which is why you get the fault. Disconnecting the battery will clear it but if you drive it about it will come back.
Now, it is probably easier to test the brake light switch (BLS) by removing it. The circuit needs to be tested with the switch depressed. Use the resitance setting on your multimeter and check the reading between the terminals. 0 or infinty indicates the circuit is open. If so replace the switch with a new one. If the the cicruit is ok then check you have voltage at the plug which connects to the BLS. If these are ok then check the fuse again (to be sure), also check the bulbs with the multimeter as there maybe a break which you can't see. If these are ok check you have voltage at the rear light cluster.
If none of these show a fault come back to us. I am sure it is the BLS, it is a common fault.
You say one fog light is not working, most cars only have one fog light at the back anyway, are you saying there used to be two and now only one works? If you are sure there used to be two then check the bulbs and fuses, I don't think it is releated to the brake light/EPC problem.
