I take it you trying to replace the sidelight bulb inside the rear panel?
Yes, remove the inner tailgate panel, peel away the plastic weathersheet, (stick it to the inside of your shed or similar so it doesn't fold in on itslef in the wind, beware the black sticky glue sticks to anything it touches!) You may need a small torch for the following, If memory serves me correctly... You can see from the side the rear of the lamp, the bulb holder twists in, but it's a VERY tight fit, with barely any room for manouvre. Whether the bulb holder can be removed without removing the entire rear outer panel depends on just how tight the bulb holder socket is twisted in.
If you do need to remove the outer panel, just look around inside for the mountings, they are visually obvious, except the twist caps in the lower corners of the tailgate! 
