Why did the fuse receptacle for my blower fan melt in my 190E 2.6?

Ok I bought a 190E a year and a few months ago, and only when I got around to trying the HVAC controls inside the car did I notice nothing was going so I read up on it and checked and there was a fuse missing from the wiring harness for the blower fan, which does it all, the A/C, the vent, and the front heater and defrost. I get some appropriate fuses at Napa and then it all worked fine for many months, until the fuse blew, oh well a box of them costs well under $5 and that is cheaper than a mechanic ripping the car apart to see what the matter is. So on I went, popping in a new fuse every couple of months or so when the last one blew until about a few months ago when the fuse holder made itself so melted and corroded that the fresh fuse couldn't complete the circuit. Since then I have been getting by with letting the manual pressure of the air as I drive blow fresh air into the car, but as the weather gets bad out here again, I am definitely going to need my defrost and heater back soon enough. I am sure when I get the time I can replace that fuse holder DIY but what makes it do this in the first place?