First of all HI. new here and my first post. My Fiesta is having some wierd electrical issues. when i use the headlight there brightness fluctuates. Also if the windcreen wipersare are on the the fan on the heater slows down. Battery is ok and its charging fine. My only thought is that it could be a rectifier

IMO first easy step is to look for water where it shouldn't be.  Fusebox, footwells, etc etc

If all dry, you could try just cleaning the battery negative earthing contact points on the chassis, and buy a new earth strap.  Most electrical problems are caused by poor earthing, however caused.

Agreed, I'd check the battery connections and the battery negative to the body connection before anything else

3 hours ago, orangecurry said:

...you could try just cleaning the battery negative earthing contact points on the chassis...

apologies as this is poor on detail if you are a newbie - by this I mean you disconnect/un-bolt the negative/earth strap where it meets the chassis, and you make sure there is clean metal for the strap to get a good contact onto the chassis.  Use a wire brush or similar, and you only need to clean where the strap connects, usually via a metal washer.  Also add in some copperslip or similar, which will keep the contact good for longer.  None of this is wasted effort even if your earthing issue is elsewhere.

Also you have said the battery is good..how old is it

What is the resting battery voltage in the morning and after started please