Police Cruiser has new alternator, new battery, new wiring, yet, it keeps dying when put into drive.

have you try checking the air intake manifold sensor could be you need to replace it or clean it using a contact cleaner

As your vehicle ages and collects more wear and debris, the computer learns to adapt to changes and keep the engine idling. disconnecting the battery can clear any memory the computer has of trying to adapt for age and wear. If there's no "Check Engine" light on and no codes in the system, try cleaning out the throttle body and reset the computer once again. I don't really know what calebsimone means by a "Intake Manifold Sensor" but the Mass Airflow Sensor (MAF) could be dirty or failed. A vacuum leak or other unmetered air can cause this concern as well.

Hello tracy you still post on here?