Trunk lid goes down all the way but the mechanism isn't catching it. I think the manual release might be stuck in the open position, but cant see it. Any ideas or related experience? Just replace the whole part?

 

Try using a screwdriver or something in the latch.

Just use the side of the screwdriver to push the latch closed. If it locks, just hit the release button or the manual release and see if it opens.
You may have to use the screw driver to apply pressure in the opposite direction for this, not sure with our cars...

If that works, you may just have to adjust the latch or striker to work.
The mechanism or striker may have come loose from use or bass or something...

Oh, one other thing... Did someone play with the rubber adjustments (black rubber studs on left and right corner openings of the trunk) for the height of the deck lid? You might need to adjust those...

 

The latch may just be cold or sticky. Have you tried to see if it's free? Maybe a shot of WD40 will help.

 

sometimes with excessive opening and closing the screws become loose and cause the latch to move around and not close properly or at all...

wiggle the metal thing that the lid connects to in teh trunk itself... its thats loose then screw that in properly... and then check the latch itself on the lid..