Your '98 has different parts controlling the auxiliary fan compared to my '96, but I believe that the method of operation is quite similar.

When the demand for the fan first appears, the controller pulses the power lead to get the fan turning. The motor will provide voltage to the controller during the "off" part of the pulse cycle, and the controller watches for this back feed. If the electrical signature is wrong, the controller aborts the start attempt, and tries again in a few seconds.

Because of this operation, a failing fan motor can cause the fan to fail to start unless you give it some help. Of course, so can a failing control module. Which is it? I doubt that there is any way to know for sure short of substitution.