Having tracked through all the advice re blowers defunct and finding nowt applicable to me ie burnt wires etc I simply unplugged the blower and unscrewed the 3 retaining screws. The blower can then be visually examined. Mine was full of fine powdery dust which was knocked out and cleaned up. WD 40'd and hey presto all 4 blowers and added bonus the AC which hadn't worked for years started working. My local garage stated it would need a full AC jobby at mucho dinero. Now off out to dinner to spend the small fortune saved from the garage pirates. 
