The Mechanics headquarter is located in the far east of Los Santos near the Pier. It consists of the repair garage and the actual HQ building. If you're a mechanic you can spawn a towtruck at the HQ building. No extra equipment is required for the job.
The repair garage is placed at the parking lot infront of the HQ. it contains two markers. To repair a car you need to tow it to the garage and place it inside the blue marker. Then you need to enter the yellow one to fix the car. The owner of the car needs to sit in the vehicle for fixing.