Injured on the job not my employers fault but the customers fault..?

I can't go into great detail about what my job is suffice it to say I drive vehicles that my employer tests for other companies. I was doing my job, and a vehicle I was detailed to had a catastrophic failure...I was injured during the following accident I am getting Workman's comp, but I want to sue the manufacturer of the vehicle I was testing they knew of the issues and hid them from us. Now because I am getting work comp I have been told that I have no case to sue by a work comp lawyer, but I am not sure he was all that competent...any ideas? I am in AZ....I can't go into details about my employer or the vehicle so please don't ask....It is doented that I am not at fault and that the manufacturer is at fault..so do I have a case against them I likely will never be able to return to full duty....