There are a few things there that might cause this...trunk gasket (have you gummi pfleged it ever? ), a cracked rubber 'grommet' holding the wires going into the lid, and then, a popular one, the taillight seal...and possibly a leaking rear sunroof drain. If doing an under car spray, consider missing grommets from your floor/wheel well (unlikely, but possible).

 

rubber 'grommet' holding the wires going into the lid

This one I'd bet. If your elephant trunk is rotted/torn/unseated thats exactly where the water will show up. Avoid the high pressure touchless washing in the meantime if possible.

 

If the grommet is cracked, there's a thick superglue that'll hold...rtv, maybe? It's a pain to replace until you rebuild it.

...and 'what is gummi pflege'? It is how you show your gaskets you care for them. Rub it on them liberally...follow directions...not to be taken internally! (We lost three fanatics in '09...like they had to be told if it's good for rubber, might not be good for humans. Idiots...and we're better off without them, I'm sure.

 

But if the grommet, I doubt the parcel shelf would get wet. Maybe have a friend hose you down while you're in the back feeling around. (Nevermind how that sounds! )

You get to the sunroof drain, I believe, from beneath the c-pillar cover--no idea how to pull it off without cracking clips...so maybe have a few handy.

realoem.com should help you find parts--enter your vin, go to town.

 

Here's something else to consider.
Is the brushless car wash the ONLY time you have an issue??
They use very powerful jets of water that would be tantamount to rain in the middle of a typhoon.

Also I'll bet the trunk latch has never been readjusted since the car left the factory. The seal has shrunk some in that time.

 

Some car washes have under carriage jets that shoot up with force. Maybe the water came up through battery box area.

 

I ordered the wiring grommet for the trunk and it came yesterday. Hopefully it's not too lengthy of a process to change. I noticed it was torn a few weeks ago but wasn't sure if it would leak when the trunk it closed, so I hopefully that's all the problem is.

As for water coming through the battery box, I doubt it. The water is dripping from above onto the felt liner on the right side above the wheel house. Sunroof drain, window rtv, or the wiring grommet seem to be much more likely. But will see what happens when I install the new grommet. Thank you dmax and 66inthesky for telling me I'm an idiot for thinking the torn grommet isn't the issue lol

 

The water is dripping from above onto the felt liner on the right side above the wheel house

And when you pull that grommet out you'll see how far back it goes under the decklid and empties out directly over the wheel well.

You may want to cut & label each wire individually then solder and heat shrink. PITA, but as long as you keep the grommet fresh and supple (303 or Gummi Pfledge work well) you prob never have to go in there again. Good luck.

 

Not sure how you'll install new grpmmet unless you're rebuilding the loom. Not so easy. I'd glue or rtv...maybe use rubber self sealing tape too.

 

I just saw your thread and another users thread about it, looks like a pain and really time consuming but should be able to get it done. Looks like depinning the connectors and reinserting them after might be the way I go, that's probably how they did it originally.

 

This is where the rain water dripping down the rear glass and entered the torn rubber boot. For sure this is the source of the leaking water.