the lens is fastened to the lamp by plastic clips on the top and bottom of the lens. The top clips are readily accessible, but I believe that you might need to loosen the entire lamp assembly just to get to the bottom clips.
(do a search on this forum for tips or pictures/URLs on the removal of the lamp assembly)
BE CAREFUL THOUGH: your plastic clips are likely brittle by now, so pry them open gently.
once you remove the lens then you can clean both the inside of the lens and the reflectors (main and fog). Of course you know that you should steer clear of those bulbs - never touch them.
the lens just snaps back into place when reinstalling.