Where is the heater control valve on a 1998 Mazda B-3000 pick up?

The heater control valve on a 1998 Mazda B3000 is located behind the dashboard, near the firewall on the passenger side. It's not easily accessible without partial dashboard disassembly. You'll likely need to remove some trim panels and possibly the glove box to reach it.

Because of its location, pinpoint instructions are difficult to provide without specific diagrams. A repair manual specific to your 1998 Mazda B3000 is highly recommended for accurate location and removal/installation instructions. A Haynes or Chilton manual would be a good place to start.