Hi there!You need to check for parasitic draw.Here is some info.Hope it helps!Thanks
When your car battery goes dead overnight, usually either the battery is at the end of its life span, or you left something on, such as a light. Occasionally something is drawing power that's not of your doing. This is a parasitic draw, and it can cause the same result as leaving the headlights on: a dead battery in the morning.
EDITERS NOTE: The original author listed this repair using a 12v test light. As an ASE certified mechanic, the best place for your test light is the trash. A test light doesnt measure voltage; it shows if enough voltage is present to light the lamp.