I just figured it out and no need to reply, I’ll post the answer here and this is for a 2015 Hybrid Avalon. Pull down the middle back seat portion and you will see a panel with key looking slot. If the flap panel cannot pull down forward, it means that the panel is locked. Use the key that is attached to your fob and insert it into the silver slot to unlock the pull down flap. Once the flap piece is able to pull down, you will see a carpeted area on the top area. Stick your fingers in there and you will then feel a ring piece that you can pull forward. Don’t yank it hard as it just need to pull forward until the trunk latch releases. Remember to put your key back into the fob so you don’t forget it in the car. Very easy to do, once I try it out. I’ll attach a picture from the instruction manual showing how to do it. Hope this information can help someone when their 12but battery dies and you need to get something from the trunk or to jump start your car from the 12vt battery located in the trunk.

 

The non hybrids are the same feature. The cord pull actually feels like a electrical wire till you know it's there and find the loop . I think you pull the loop sideways to the left or the right?