David,
Assuming that your car is wired correctly, meaning that no additional load points have been placed on the circuit (the part downstream from your fuse) AND
all or most connections to that circuit are clean (dirty contacts including the fusing point draw a greater load) AND
the integrity to the wired points on the circuit is intact (corroded or damaged wiring will draw a greater load) THEN
...the thing to check (sorry for the bad news here) is your blower motor.
DC motors draw current in relation to the load on the motor shaft. So, it is possible you have something internal to the blower motor that is causing an additional physical load on the shaft; think a bad bearing or something worn out. Then, the motor is drawing more current than it is supposed to and the fuse blows.
It is possible to have your motor rebuilt once you get it out, (not a trivial task I'm afraid) but our vendor here, Bud's Benz has sourced suitable replacements. Part number is X248-011B @ $189.00
I should add that there are such things as "impedance protected" or current limiting motors which are typically used in electronic equipment cooling applications (think the fan on your PC) that do NOT draw additional current in relation to load, and can suffer a bad bearing w/o blowing a fuse. Alas, our blower motor is not in this category...
