Where is the blend door on a 2000 Dodge Durango?

The blend door on a 2000 Dodge Durango is located inside the dashboard, behind the climate control unit. It's not something you can easily access or see without removing parts of the dash. There's no single external location. To reach it, you'd need to disassemble portions of the dashboard, a significant undertaking best left to experienced mechanics unless you're very familiar with automotive disassembly and repair.