It's right there in the next room, but it's no wonder you didn't figure it out.
Through the door is a hall filled with light posts. Listen to them by pressing down in front of each one until you find the one that works. The only indication of success will be the water resetting itself.
Now, you're in an identical hall. Do the same thing, listening to every post.
Then, I believe you'll be in a hall with light posts and "bodies", but the lights are out. In this area look for a section where 3 bodies are below the tops of the light posts. Jump up to the top of the posts and press down. When you get the right one you'll be in a lit hall again.
Do not exit before finding a yellow key.
Next thing to do after getting the Yellow Key is to get the Blue Key.
For the Blue Key, go back to the parking garage "massacre" site and poke around for new secrets.
Keep going until you start retracing steps, then try and figure out the puzzle.
I'm not sure how you're SUPPOSED to solve this puzzle, I solved it by using the KS+ Map cheat. However, I believe the wallswims and obviously wrong walls are glitches and not part of the puzzle.
Go back to the parking garage "massacre" site, and activate the lights above the rightmost bodies. You'll know it worked if everything goes dark and a passage in the floor opens.
Continue through the passage until you reach the Yellow Key box.
Continue through the passage until you are retracing your steps.
Ignore any wallswims or obviously wrong non-solid walls; these are glitches, not part of the puzzle.
Run back and forth between the screen at the bottom of the shaft, under the cars, and the screen next to it, under the doors.
Start under the cars and run back and forth until you've been on the car screen a total of four times (including the first), then on that fourth time, run to the right, under the doors, then into the hallway with the doors and the clock.
You'll know when you got it because the hall with the doors and clock will darken as you run past the middle door.
I'm not sure how you're SUPPOSED to solve this puzzle, I solved it by using the KS+ Map cheat.
The purple key involves the "original" killer of the parking garage massacre from years ago.
Look above the white car again.
He's goading you on! What are you going to do about it??
* Jump up above the White Car in the parking garage and go into the room with the mean monster who has "bulletproof" skin.
* Jump up to his face and press down.
* Continue jumping up to his face and press down until a new passage opens up.
The purple key doorway is in the creepy "underwater" area, under the island with the green crocodiles and the green trees. It's got those big black floaty bodies where you have to climb along the ceiling.
If you find yourself climbing up the long narrow vertical passageway with the door at the top, you've gone too far and need to go back down.
Explore the area at the bottom of that vertical passageway.
* At the bottom of that vertical passageway, there is another vertical hidden passage 4 blocks away on the left.
* It does NOT open like the others do, it's just a nonsolid wall.
* It's also a VERY tricky jump to make. Recommend using that secret "rat room" save just to right of the first floating body, in the ceiling so you don't have TOO much backtracking to do.