• Any working webcam,
  
• Opentrack software (free)

• Download Opentrack software from here[github.com]
  
  
• Run opentrack.exe
  
  
• Click Profile, then Create new empty config. Name it as you wish.
  
  
• Select input, output and filter as marked below. I tried almost all options and found this is the best stable configuration.
  
  
• Click Options button.
  
  
• Select Tracker tab and make sure your webcam is selected. Leave all other settings as default.
  
  
• Select Output tab and change settings like below.
  
  
• Select Filter tab and and change settings like below.
  
  
• I strongly recommend to assign shortcuts to start/stop tracking and centering. It is very usefull while in-game. To do this, go to Shortcuts tab and bind Center and Toggle tracking to any key on your keyboard, gamepad or even racing wheel. In my setup, I assigned both to my G29 L2 and R2.
  
  
• Click Mapping button.
  
  
• Click each tab except Roll and adjust curves exactly like below. After I spent hours, I found this is the best stable configuration. Left click adds new point, right click deletes point.
  

Here is the explanations of each term.
Yaw: Adjusts the sensitivity of looking left and right.
Pitch: Adjusts the sensitivity of looking up and down.
Roll: Adjusts the sensitivity of rolling your head left and right. Not supported by ETS though.
X: Adjusts the sensitivity of moving your head forward and backward.
Y: Adjusts the sensitivity of moving your head left and right.
Z: Adjusts the sensitivity of moving your head up and down.

• All done! Now click Start button and see if it works.
  
  
• Now you can start ETS2. No need to do additional settings in game. Game automatically detects opentracker. Enjoy watching around while driving.