try pulling off the connections to the head lamps, and see if the fault is still there, with the wires still off turn on the head light , is the fault still there, that tells you it in the loom, replace one headlight connector an turn on the power,then check the fault, remove it and put the next one on m, check the fault., and finally replace them both , this should tell you if it one side or the other or a common line between both headlamps thats the cause

 

@JUKE179r daytime running is just headlights but it was a good idea, after market modules can do weird things to your headlights.

Sorry if this seems obvious but maybe it will tweak something for someone :

Do you have rear fog lights and if so is that switch on, left side of dash right below the front fog/driving lights

What it seems you have is the rear running lights circuit tied into the brake light.
So instead of just the taillight circuit being turned on with lights you are getting both tail lights and brake lights
Is it possible you have the wrong bulbs ?