2012 GMC Terrain - battery drain

something is draining battery over night, then needs charge

How old is battery? That is constantly being drained?

Ask tech to monitor battery drain with current meter while removing fuses. Should be able to demine at least which circuit it is. A common cause is a faulty driver’s door switch.

check to make sure there are not any lights staying on like dome light, glove box light.
have your battery checked to make sure it is still good. most Autoparts stores will do this for free.
make sure the brake lights are not staying stuck on. a faulty brake light switch will cause this.
check for a parasitic draw.

The BEST Way TO Perform a Parasitic Draw Test - YouTube