Is it permissible to take money for guiding customers to purchase cars belonging to the company, knowing that this is not part of one's job duties, and is it permissible to give a portion of this money to the person in charge of cars at the company?

If your guidance to the buyer was not during your working hours at the company, and it was not part of your job responsibilities there, then there is no blame upon you for taking the commission. As for the sales manager at the company, he is not entitled to a commission on the sale, and his demanding it from you or from the buyer is an unlawful taking of money.