start by putting power to the motor to make sure that it really isn't the
motor.
I have had this problem due to intermittant connections due to oxidized
copper in the central electronics - wiggle everything there with blower
selected on - it may start running

to check relay, use an ohmeter and clip leads - to check ballast resistors,
use ohmmeter.

If motor runs when powered, then just trace the circuit backwards - the
schematic is in the service manual