Dealing with insurance companies

Looking for any advise or input on dealing with a insurance company on a classic car. My 71 swinger is insured through a classic collector policy for a agreed 12k. A few months back it was hit in the rear by a guy in a 1 ton chevy truck, his insurance accepted the fault, but the issue is they don't want to pay a fair amount....what's the best Avenue to pursue a fair settlement?