New battery, new alternator, new voltage regulator, still discharging? Help

 

pre heater stuck on,take heavy wire that goes to preheater and disconnect and see if that works

 

Just disconnect the battery first.

 

Unplug the voltage regulator. With the ignition switch on, you should have battery voltage on one terminal of the plug and slightly less on the other terminal. Next you should have battery voltage at the alternator on the big terminal.