My question involves collection proceedings in the State of: California

I rented a car while I was abroad and got into an accident. At the time of rental, I did purchase their loss damage waiver insurance policy with a deductible. They have been trying to collect the full cost of repairs to the vehicle as opposed to the deductible. The rental company originally sold the debt to a foreign collection agency which tried to collect and violated several collection laws, but I am not sure anything can be done about that since they were not based in the US.

The debt was subsequently sold to a US-based collection agency, and they have since been trying to collect from me. I have stated time and time again to both the collection agency and the rental company that I did not breach the original contract terms and that I was only responsible for the deductible. The wording in the rental agreement is purposefully vague, but the conditions under which the accident occurred should not preclude the insurance from paying out.

They reported the debt as a collections account on my credit report. I have since disputed it, but the rental agency insisted that I was responsible for the debt when I asked the collection agency to validate it.

I am not sure what to do at this point.

If I am not responsible for the debt because I have not breached contractual agreements, how do I get the collection agency off my back?
What is my course of action to prove that I did not breach the contract?
And to whom must I do this? Do I need to take legal action?

I appreciate your help in advance, and please let me know if you need additional information.