If all your drains are clear, I'd bet good money on the windshield seal allowing a leak. Even though the seal material still seems pliable, it can shrink dimensionally and pull away from the metal lip of the windshield opening just enough to let a thin sheet of water get through.

Get out your wet-dry vac and remove all the moisture that you can. Also, lift up the rear-floor carpet and remove any water that may have migrated back there so you are not surprized later ...they may feel dry on the surface but could be wet underneath.

The seals are not prohibitively expensive and any glass replacement shop can do the job...some are more willing than others and the labor will vary from shop to shop. I suggest replacing front and rear at the same time but that is because both of mine needed it after years of South Texas sun. One caution, there is a risk of breakage especially if there are any rock chips in the glass, but if the installer is extra careful and is not rushed the replacement should happen without a problem.

The dual seal replacement on my '82 240D was needed after 20 years (and 300K miles) and the labor was 100 bucks for installing my mail-order OEM seals. Good fix