Getz high level brake light
 

Hi, Model Getz 1.1 gsi Dec 05, 55 reg.
Can anyone tell me how to remove the cover on the high level brake light unit in order to change the bulb (I presume it is a bulb and not LED).
The plastic cover appears to be fixed solid and I am reluctant to force it in case it breaks.
This light unit is the only one not covered in the bulb changing part of the handbook.
Thanks in anticipation.