Is it permissible for an employee whose salary has not increased for six years, despite unfulfilled promises, to use the company car for his personal use in exchange for an acceptable salary, especially since he performs additional tasks?

You have the right to demand a salary increase, but you do not have the right to take it without the company's knowledge, as this is a form of theft and a betrayal of trust. You must adhere to the salary agreed upon with the company. It is not permissible to exploit company property (such as cars, etc.) beyond what you have been authorized to do, and you must adhere to the terms of the employment contract; as the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions."