I won't go into the many reasons why that is a bad idea but I will just point to one - the insurance company will tell you that the dealer wants to buy the car cheap so they have a vested interest in giving you a higher diminished value figure.
Diminished value advice from an appraiser
Rules about diminished value - In every state except Michigan, you can make a diminished value claim against the insurer of a third party that was responsible for causing the damage.
In only Washington, Kansas and Georgia can you make a diminished...