rear lights - bulb sockets are melting

The rear lights on the van keep going out. I check the bulbs and they are smokey. Then I checked the bulb holder and noticed that the contacts are bot shiny and appear melted. I changed the whole bulb housing and it happened again months later. Changed it again and now months later,  the lights are intermittent. Seems like some kind of defect in the design. Same thing happened once on my 2001 montana. Anybody know whats going on and how to permanently fix the problem?
I have both a 2001 and 2003 montana and both have experienced this problem.