Water in trunk??

[pulsejakob]

What would cause water to get into the jack compartment?!?

[estoguy]

How is your trunk and tail light gaskets?

Also, check your antenna housing. Could be cracked or something.

[agent]

In my case, a hole that's rusted through between the jack compartment and the wheel well.

[DesertBMW]

hose from the sunroof also goes there, remove the cover and inspect

[mitchlikesbikes]

mine was a loose taillight, that seems to be the most common cause

[ELVA164]

Sunroof drain, sunroof seal, rear window seal, antenna grommet, tail light gasket, trunk brake light if you have one, trunk seal.

[audiquattrot]

Also check the 3 grommets that hold the exterior c-pillars in place. Mine was leaking like crazy from there after the car was repainted. The body shop didn't put new ones in there when reattaching the pillar

Same w the gasket around the vent that sits on either side behind the rear side marker lights

[redlightpete]

Mine leaked from where the upper left corner of the rear window seal had shrunken and pulled away. I sealed it with some RTV.

[whitebulat22]

I've got the same exact problem, my left rear window isn't sealing so half of the recent downpour at my house went in that jack compartment.