Look here resisters that break down is a common problem,and fixing them can be done on 2 ways:
Changes the heater motor this takes quit a while because half the interiour needs to get out.
Do it the way how it is showt in the pic in the link,and place some aftermarket resistors.
opel, senator, monza, omega
If you have a Senator A 1978-1986 thats the way to do it.
If the fan is going without making noises i would place the aftermarket resistors,i have changed a heater motor twice now in a Senator A1 and a Monza A2,(both have kind a the same interiour) i would never do it again if the resistors break down.
If you car has AC its a complete diffent story,thats very easy,but those resistors are hard to find.