Here is the form of what my firm uses when it comes to a vehicle preservation letter. Remember, this is a form, not legal advice:
Our firm has been retained by XXXXXXXXXX to represent him in an accident which occurred on XXXXXXXX.
It is my understanding that the vehicle described above is currently in your possession and is being stored by you or a representative of yours. Because this vehicle may be evidence in a liability lawsuit, I ask that you safeguard this vehicle for that purpose and not make any repairs to this vehicle, remove any of its parts, make any modifications or change its condition in any manner.
If my request is inconvenient to safeguard the vehicle, please call me within the next twenty-four hours, so that I will know whether other arrangements need to be made. Should you decide to sell, strip, trade, or transfer the vehicle, please contact me. You are prohibited from taking any such action unless you have my express permission in writing to do so. In addition, if any insurance company notifies you concerning the disposal of the vehicle, please contact me immediately at the above number.
If you need further information from this law firm, please do not hesitate to call me. I must tell you that it is very important that this vehicle be preserved in its present condition. Any changes to it may be deemed spoliation of that vehicle.
This letter usually gets the attention of the recipient.
Again, recall this form is one I use in Georgia. Please don't consider it legal advice. Thanks!