Is the actuator inside the heater core housing in my 98 E 350?

 

There is a trapezoidal hatch in the middle top of the dashboard. Pried it open and there are a couple of actuators.

Before re you get too carried out with the actuators. Are you having problems with your vents diverting to your feet? Specially under acceleration? Search for how to fix vacuum reservoir/ check valve.

 

Vents seem to be working fine. AC blows well but I got no heat! I don't know if the dash is different, but it's an E350 engine in a Tioga Motorhome.

 

In my '94 E150 there is a blend door in the molded case that surrounds the heater core and duct work. It's controlled by the "Cool-Warm" slide control on the dash group. If it's stuck in the "outside air flow" position, you wouldn't get any benefit from the heater. Some are controlled by a wire in a sheath (like an old choke control) and some are electronic with a servo motor. Mine was electronic and intermittently working. Turns out that the solder connections on the slide control had developed cracks. A quick reflow with fresh solder fixed that. Sorry that I don't have a manual for your '98 but systems in trucks don't change design as fast as you see in cars.
electronic control