Brake Light bulb keeps burning out

[glenmore]

1999 C280, about 95m.

My daughter has had this car for about a year now, and I've noticed that it keeps burning out the brake lamps. I am pretty sure I am using the right P21W 12v bulbs. The housing where they plug in is embossed P21W.

On further inspection the plastic bell housing are also melted, both sides.

Bulb 1156 can also be used. I'll try those.

Any ideas?

Thanks.