Changing Number Plate Bulbs on Bailey Orion 430-4

Hello one and all! I want to change the bulbs in my 2011 Orion 430-4 number plate light  (there are 2 of them) modules. It shouldn't be a problem, except there are no obvious screws, only 2 plastic looking grommets on each light. I don't want to force anything as plastics today are fragile to say the least, but before I try anything, does anyone have first hand experience at getting at the festoon bubs within?  The other possibility is that the light units slide along in slots until the light releases? Does anyone have the answer? Thank you in anticipation!