I do not know if this what is wrong with yours but on my E300 the wire clip at the rear of the light that holds the bulb cover on had broken. This left the light assembly open to moisture. That moisture will certainly make your bulbs short lived. I did not want to spend $250.00 on a new light assembly so I drilled a new hole to attach the wire retainer. I then had to disassemble the entire light (pulled the clear part of the lens from the grey back). Used a blow dryer to dry all the moisture then reassembled and all has been dry sense. There are rubber seals on the cover that the wire retainer held to protect the bulb connections. If you attempt to disassemble the light you will have to grind or otherwise break the grey epoxy seal for the clear part of the light. Then you can carefully release the clips that hold the clear lens on. It is tricky but certainly doable.
2006 E350 98K miles
2013 Ford Explorer 15K miles