This is for a specific Actuator that controls the vents from MAX AIR all the way to DEFROST. I believe more than one Actuator does this but this is the one that is notorious for going bad. It is located on the side Plenum above and right of the gas pedal tucked in neatly behind a engine control...
Sounds like the blower resistor or the blower motor us gone, plus the actuator as 3L71 indicated above. Resistor or blower motor are easy to change. Just hook the blower to 12v and if it spins well, it's the resistor.
I just fixed my '04 - the fan speed only worked on "3" and "4". Speeds "1" and "2" did nothing. The fan ran at the appropriate speeds on 3 and 4.
There were a lot of older discussions regarding the resistor, but everyone who had a problem with it said that the fan either didn't run at all, or went to full speed at the lowest setting. I suspected the resistor, bought a new one, and it works fine now. Both the resistor and fan are really easy to get to, they are under the glove box.
Also, because different versions of the Titan had auto climate control, whereas mine was the "old fashioned" 4-speed manual, there are two or more different resistors. I simply pulled mine out to see the part number and ordered that one.
Here's a good video on where the fan and resistor are, and how to check and replace them: