I would bet those white tabs are plugs. The reflectors on my trailer are snap on, so I can not say for sure. I would grab a very small flat blade screw driver or an exacto knife and see if one of those tabs can be popped out. We really need to find a source that had exploded assembly drawings of these trailers. I am going to start searching and see what I can uncover. In the mean time I'd try a little prying and see what happens. If you take it slowly and it starts to crack, use a little silicone to seal it up. I PM'd you one other suggestion.
2006 Nissan Xterra
B&M Trans Cooler
2011 Nomad 206
Panasonic/Pioneer/Kicker ~ Audio/Video System
Days Camped /2009 / 33