Using class methods in another file not working in WordPress

I’m trying to include a class in PHP and to use the methods from this class in another file. My code looks like that (small reproducable example). By the way: I’m working on a WordPress environment.

Main file:

include 'class-file.php';

$cars = new Cars();
var_dump($cars->getCars());

File class-file.php:

class Cars {

  public function getCars() {
    return "No Cars";
  }

}

I get the following error:

Fatal error: Uncaught Error: Call to undefined method Cars::load() in /..../wp-content/themes/builder/includes/elements.php

Seems that my class instance is connected with another class from the theme. But why? Can I disconnect the class from any others? If yes, how?