This objective is revealed after interrogating the second pusher. Drive around and look around for a red vehicle in your minimap. Follow the vehicle closely to trigger a Tailing sequence. Follow the car carefully until you reach the hideout.
You can kill the bagman as soon as he gets out but if you made some noise, you’ll have to deal with a lot of armed guards inside the warehouse where the stash is kept. However, if you're using a silenced weapon, you can take out the bagman silently as soon as he exits the vehicle then quickly kill the lookout before he can draw his weapon and alert everyone. You can wait for him to go inside, kill the guard outside then sneak inside so you can throw a grenade to get rid most of the opposition without getting endangered by getting overrun.