Air and heat fan switch 99 chev. silverado 3500

my dash fan switch goes out every 3000 miles somtimes less first the lowest setting then the rest ...then only max works now max only works when it wants too..ive had the dash switch replace and a resister block which controlls the fan speed ...but the block was fie ..the switch back melts its pretty cheap looking but what makes it get so hot ..it should have a relay that controls the voltage contacts when activated..maybe a fan gets overloaded but it woks fine after a new switch gets installed...