someotherguy
+1y
88-98 all trucks in this body style, when equipped with A/C, use electric actuator motors, not cable sliders. However the motors may be different once the swap to the 95-up style controls came along - I've never bothered to compare part numbers; I can tell you that around 94 the size/shape of the blower motor resistor changed so the hole in the duct box changed to match it and the best way to accomplish that dash swap would have been to swap out the whole box, which would have brought along all the newer motors too.
Heat-only trucks had cable sliders but obviously you'd run into trouble trying to use any of these parts with A/C - you didn't tell us though if your truck has A/C. Even the late model heat-only trucks I've seen use cables; was gonna do an A/C conversion for a guy with a 2000 C3500 utility truck but he backed out when he realized just how much needed to be replaced including the engine harness (had no provisions at all for the A/C components), duct box, adding the condenser, cutting the holes in the firewall for the evaporator lines...
Anyway what Trevor said, the arm could be broken. I've also seen more than a few of the later model setups where the flap breaks inside the box, which sucks. If it's A/C equipped with electric actuators - the vent position actuator is the only one that has linkages on it, they get gummed up from the factory lube hardening and they quit sliding, making the functions work incorrectly. Check it out; this is the one on the far left of the duct box, which is just above the driver's side of the transmission hump. Also, control board being bad can cause this.
That it only blows air on the highest setting is either a bad blower motor resistor (most #1 common reason for this), or they left the old duct box in there and didn't hook up the resistor or the blower speed relays correctly (the number of them and the arrangement changed over the years, especially because 98 has 4 speeds and 88 has only 3.)
What's this about the brake lines?
Richard