Hi all i have an issue with the lights on my 2004 Mondeo left work last night drove home to find i had no rear running lights but head lights were on.

I have no side lights or number plate lights my main & high beam lights work break &  reverse lights work and indicators work i checked the fuses in the car they were fine but i have found fuse f21 in the ebgine bay blown i replaced it turned lights on and it blow again according to the manual it's the light switch found my switch be faulty?

have you installed anything in your boot? Sub and Amp etc.?

I had an issue where I installed a Amp int he boot on my car and it kept blowing my light clusters in the same way. Anything that had a constant current going through got fried. Indicators and brakes were fine. 

Check for burn marks under your interior lining, burn marks on any metal to metal surfaces, even if they're tiny gaps. Check your seatbelt anchor points and rear wheel suspension points where they contact the body.