How do I calculate the diminished value of my car after an accident?

My new car was recently in a very bad accident. The car is a Nissan Altima SL and has 7000 miles on it. I did some after- market upgrades to the car, (tinted windows, sound system, satellite radio). The damages to my car totaled $14,000 in repairs to the front end and I need the insurance company to compensate me for the diminished value of the car. How exactly does one calculate diminished value. I paid $29,000 for the car 4 months ago.