How do I fix a seat belt that won't click together?

It's usually a bent or missing spring in the buckle. You can't buy just the replacement part. Best bet is to find a used one at a junk yard. You can use an online service like car-part.com

When it comes to safety equipment I wouldnt mess around with a "quick fix" you might get it to latch now, but it could fail again, in an accident. I would replace it. That doesnt mean you have to go to the dealer and spend a ton of money. Call your local salvage yards or look on Amazon and pick on up on the cheap!

Make sure the latch is going in right , flip it over ,or in the correct pocket ,

I'm going through this myself on my own GMT360, just need to unbolt the female part and replace. I'm going to junkyard myself.