This is apparently very common, and high watt bulbs are not even required to get melting. The sockets are simply underbuilt, which is very unbenzlike.

The replacement sockets they sell at Autozone (less than $5) are much beefier than the stock ones.

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1986 300E 5-Speed 240k mi.

There's nothing like a nice set of Euros as well as high-wattage lights (80/100 h4 and 100w h3 fogs) It's a combo I've run on 2 different models for a total of 6 years with no problems at all. The bulbs can be had cheaply at a snowmobile/motorcycle shop. Much more cheaply than at a car tuning shop.