So fitting a F1 style brake light under my diffuser ,?tried to get in at the rear of the brake light through the boot, this is a big job so was wondering can I tap into the fuse for the brake lights some how, will it still work this way

 

No. The fuse box has the buss that supplies the power to all the fuses. The circuit is completed downstream of the fuse box at the brake pedal contact switch before carrying the current on to the light.