Electric fan will not turn on

This is not a typical problem. However, when dealing with such different systems that shouldn't have anything to do with each other but somehow do, I'm thinking:

Grounding
Rodent/water damage
Wire chaffing
Wiring harness overheating or other electrical faults
"user" mods
Bad PCM or GEM.

The defroster is grounded via the cluster. The cluster is grounded via G204. G204 is a very important ground located behind the center console. It's shared with multiple devices including the radio. G204 is frequently messed up during a radio install.

Further, the defrost relay is located behind the center console. Which begs the question about any recent work.

Normally I would recommend starting by checking the CCRM. In this case, I think you should inspect the PCM connector for evidence of water damage.

Next, disconnect the coolant sensor electrical connector. Does the fan run? If so, this says that the CCRM is basically OK. This also proves the fuse powering the fan.

As for CCRM tests, access the CCRM by removing the plastic fender liner. With the key off, measure the resistance of CCRM pin#15 (BK/WH) back to battery negative. Should be low. Post.

With the key on, ground CCRM pin#17 (LG/VT), the high speed fan should kick on. If not, ground CCRM pin#14 (DB) AND CCRM pin#17 and see if the high speed fan kicks on.

If grounding both 14 and 17 does not cause the high speed fan to run, focus on CCRM problem.

Based upon what is found with the above tests will determine future tests.

Is there more than 1 temperature sensor? There might be one for the guage and one for the fan relay.

The 96-99 4.6 V8's model years have two engine coolant temperature sensors.

The 99-04 has a single engine coolant temperature sensor. The cluster temperature reading comes from the PCM. The cooling fan is controlled by the PCM. The cooling fan logic is actually fairly complex. For example:

is the AC on? If so, turn on the low speed fan.
How hot is the motor? Two step process of low fan. If temperature continues to rise, turn on high speed fan.
Is the motor overheating? Go into "limp in mode".
How fast are we going (turn off fan above 45 MPH unless overheating).
There is logic inside the CCRM to prevent cooling fan short cycling (min on time).
What is the temperature outside? When it's cold outside, the fan will run from time to time to prevent snow/ice from making visible steam.