Not sure what the rated blower motor current is, but it is probably up in the 5-10 amp range. Those resistors are pretty high wattage parts, I would wager the stock coils get pretty darn hot under load (like maybe to the point of glowing red hot), they are made of nichrome wire from the looks of it (same stuff used for heating elements).

I know on my VW, the blower resistor is a big multiple tap wire wound unit, probably about a 50 watt rating judging from the size of the part. Might be the best way to go with a unit like that as you can move the tap points around on the resistor coil to adjust the blower speeds. On the VWs, cars with A/C have the small open coil resistor shoved into the blower duct and those have problems blowing out as well. On the cars w/o A/C, they have the big wire wound ceramic resistor out in the blower air intake under the hood and those never fail.