did you check battery condition and alternator condition? as you said that with more electrical load the worse it gets.

Also break vacuum could be an issue you can try this;
start the car then turn it off, press the brake pedal a few times till it gets hard then press it and keep it down. with pedal pressed turn on the car - brake pedal should move further down due to servo assistance. If it does it means servo works, let car idle for a few seconds, then turn off. Leave it off for like 2-3 minutes then pump the brake pedal again. If pedal starts soft then gets harder it means that servo and no return valve are holding the vacuum so good. if It doesn't means you have a leak from servo or no return valve, if both are good means leak is from pipe or other side,

Is that the 1.3 fuel injection or carb? i believe carb take vacuum from inlet manifold? injection same?
I had several mk3 fiesta's but mine where diesel however servo is a fault on them and on mine they all failed - issue is master cylinder leaks fluid into servo no one cleans it and it eats everything up.