I used to have a tiny bit in my headlights now and again, so grabbed a couple of these big dessicant sachets from a new iMac box at work and put one in each headlight inside the plastic cover.
http://www.ebay.co.uk/itm/100-g-Sili...-/321090787338
I had some in the foglights after changing the bulbs and after 3 months it hadn't gone so washed them out with alcohol and put them in a low oven for 15 mins. I noticed the rear cover gaskets were a rather dusty and dry due to caked on soil, so cleaned them up and put a feint smear of silicone spray on the gasket seal with a cotton bud. Maybe your headlights have this problem with moisture wicking past the seals of the covers?
After all that, now I have both nearside tail lights steamed up.
