Totally got through the game. All levels are unlocked.

First step
You need to download and place the save files in the game folder.

Save game data location:
<path-to-game> \data\SAVEDGAMES.ARS

Example:
C:\Program Files (x86)\Steam\steamapps\common\Carmageddon2\data
or
D:\SteamLibrary\steamapps\common\Carmageddon2\data

Download:
https://drive.google.com/file/d/1S4-aHWbMGPAY9hM2WFApYnguhfVksj9n/view?usp=drive_link

Second step
Start the game and go to the settings. Change “Auto load” to ON.



Third step
Next go Menu -> Load game. And select the upper save file.



You can switch between levels using the red arrows on the main screen.