2015 suburban LTZ - every time we get a nice steady rain, the drivers backseat seatbelt leaks! Anyone else have this issue? How to fix?

 

Welcome to the forum. If you have a sunroof, check to make sure the drains are clear.

 

If the drains are clear, one or more drain hoses possibly came loose. Sunroofs leak so they have a pan that collects the water and drains it through tubes to the outside of the vehicle. My 2007 leaked at the belts after heavy rain. It happened infrequently so I didn't investigate.

A couple years ago, a tree fell on it while my wife was driving. The sunroof wasn't salvageable and had to be replaced. When we got it back, it leaked worse than before. Since the roof was replaced, I thought it was leaking at a seam. Turns out, 2 of the tubes came detached. The rubber connectors no longer fit snug and came off easily. used zip ties and pulled them snug enough to hold them in place. It's completely dry now even in a monsoon.

The front two run down the window pillars. The back two run down the door pillars. Before you start pulling off trim pieces, you should be able to pull the weather stripping back to where you can pull down the headliner a bit to see if they are detached. If they are detached, you'll need to remove inside pillar trim pieces to get to them.