If the blower motor randomly and intermittenly quits and does
not work, chances are good the blower motor itself is faulty, if
the blower motor does not work on all fan speeds from low through
high, it most likely needs a blower motor resistor.
In my '95 cherokee, the blower motor resistor is under the glove
box. I assume it's about the same on a 1990. I posted instructions
(with pictures, so at least you'll know what you're looking for)
for replacing it on www.timsjeepblog.com.