Drivers Seatback Will Not Lock In Upright Position

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My brother in law borrowed my truck to tow a car hauler and the drivers seat broke. The seatback stays tipped forward unless you're sitting in the seat and then it falls all the way back and won't lock. Any idea on what might be broken anyone had anything similar happen? Is it a spring in the lever that tilts the seatback? Can I get to it without removing the seat?

 

thanks in advance for your help!