To determine who is responsible you can first contact the tow truck company and see if they will assume responsibility since the car was being handled by their employee at the time of the accident. If they do not want to take responsibility next you can get your car insurance company involved.

What you insurance company would likely do is assign a claims adjuster to the case. This person would then investigate the claim and make a determination of who was at fault. If they believe it is the tow truck company that is responsible typically your insurance company will try to help you get your damages fixed by that entity then. If they determine due to it brakes not working the responsibility is on you then you will likely have to put a claim in with your insurance company.

Car insurance groups can be found here.