How do i get my money back that was seized during a arrest?

This sounds like a case of civil forfeiture, which is very onerous on people whose assets have been taken. The prosecutor does not need to prove that you committed a criminal act, only that there is probable cause (a very low standard of proof compared to something like beyond a reasonable doubt) that the money or other property was somehow connected to a car. If your money was in the car, and it may somehow be inferred that it was linked to a criminal action, even if it was of the other people in the car, the prosecutor may have enough to justify the seizure. You definitely may need an attorney to fight this. Be prepared that if the prosecutor does decide to return the money, they will also often require that you agree to waive any rights to file a suit for damages.