Immediately file a claim with your own insurance company. You will be responsible for your deductible and they will pay for any damages over that amount. Your insurance company will then do the leg-work to track down the other driver's insurance based on the phone number and such information on the defendant as the police may generate. Your insurance company will obtain reimbursement of both what they pay out and your deductible amount, then refund your deductible.
If the defendant driver is uninsured, you may pursue a claim in "small claims" court. For this, go to your local District Justice office and prepare a fill-in-the-blanks complaint using the information and evidence obtained from police and your insurance carrier. Hire an attorney for the small claims court hearing if the extent of your damages/deductible warrants that expense. Good luck to you.
I suggest you call the police and let them do the investigation for you. Or you may wish to hire your own investigator and let them find the girlfriend and the truck owner through her. Either way, you have the handwriting of the truck owner admitting he/she hit your car, and that should be enough to start and prove a law suit for damages.
You may also want to hire a lawyer to file a suit for you, but that will depend upon how much your damages are and whether it’s worth it to you.
I suggest that you file a formal complaint with the police department and ask them to further investigate and identify the culprit and his insurance company.
Also, if you have uninsured motorist coverage which pays for property damage caused by uninsured drivers, file a claim with your own insurance company and provide them with the information you currently have.