lj, careful with the wire. Some of the drains are connected by rubber hoses. A safer bet is weedwacker line.

Good news is: There are drain holes in both trunk wells under the plastic liners and also in the spare tire well. They should have removable rubber plugs.

Bad news is: The water incursion is probably from a rusted out rear windshield channel.
It was on my '83 300CD.

Medium news is: Unless you are a body welding wizard it is a job for a body shop to rebuild that channel. It is not as expensive as you might think. It cost me a total of about $400.00. That included a pro glass shop windshield R&R and a new gasket. The coupe rear windshields are rare in the extreme so I took no chances.