hi  I have all rear lights both sides but just one side brake lights when i put the brakes on . 2007 Fiesta ,1,25

Have you checked the bulb?

If it's not the bulb, the light cluster track is probably corroded, common fault, just needs cleaning with a small wire brush.

All the bulbs ,three i think work on the side i am having bother with They just dont "brighten" when i put the brakes on like the other side .Thanks i will take the cluster off and see if there is any corrosion

The bulb doesn't actually brighten when you press the brake pedal, there's two filaments inside the bulb (one for tail light and other for the brakes). One filament has probably broken requiring a new bulb.

sorted corrosion around the back light cluster

Hi I'm having a similar issue with my rear lights I've looked and can't see and corrosion and changed the bulbs when I brake my side light goes out on one side and the other brakes when I drive off the light that brakes goes out and no side light but I've noticed today that my rev needle when u brake it flashes red I'm stuck for ideas to solve this can anyone help me please 

Poor earth connection. Check the the resistance of the earth wire from each of the lamp holders to a good earth point and you will find that one of the wires has a high resistance.

I agree with unofix. Common problem.  Some cars suffer more. Mk3 escort was a shocker for this due to corroded connectors. Quickest way to fix or test the theory. Get some wire (not too thin) strip some insulation off each end. Push one end down the side of the metal part of bulb in bulb holder (but not so far down that it reaches the terminal at bottom) fix other end to car body, eg undo a mounting screw / bolt and put the bare end of wire under the head and do up. Depends how professional a repair you want. Recently fixed my friends bmw325 rear lights this way. He was well pleased. He envisaged a bmw size bill.  (I did it free of course)