57 Corsa D Break Lights not working?

Hello I am wondering if anyone can help me, I own a 57 plate Vauxhall/Opal Corsa D 1.3CDTI 5 door. When I break the top strip light on the tailgate illuminates, however, the brake lights in the two light clusters on either side of the rear of the car do not.

This is what I have tried so far:

I have tested and replaced the bulbs, the bulbs were fine however I still replaced to no avail

I have tested and replaced the fuses, again the fuses were fine but replaced anyway, no avail

I have taken a multimeter to the fuse box and there is a consistent reading of 12V regardless of the break peddle being pushed or not. I am not sure this is normal behaviour but I assume it is.

I have taken a multimeter to the light cluster contacts and no voltage could be found on the brake light connections. (All the other lights are working fine and showing a voltage at the connections)

I have removed the plug from the brake light switch, connected to the break peddle, this cause what I presume to be a safety feature and activated the top strip break light but nothing else. I then jumped each of the contacts to each other to see if that would do anything thus indicating a break light switch problem but again to no avail, the top strip light remained on the whole time the plug was removed from the switch, however no combination I tried caused the other brake lights to illuminate.

Thank you in advance