When it rains the back seat floor on the driver's side of my '82 300 diesel coupe turns into a small lake. Only the floor gets wet- even when the car is parked. The seats are dry, the windows don't leak, and no other parts of the interior gets wet. Is there an odd drain somewhere? Anybody else dealt with this?

 

There are several possible sources of water.

One is thewindshield - even though you may think it's OK. Water gets around the rubber seal, runs down the inside of the firewall and eventually makes it's way to the rear floor.

Another area for concern is thedoor seal. Check the condition of yourrubber and also check the door bottoms which are notorious for rusting out. Hopefully there is no unpleasant surprise for you here.

I've heard that the sunroof drains may be a factor but cannot say for sure since I don't know where they exit the chassis - maybe someone on the forum knows about this.

Water may also enter from the undersidenear the rear jack points. If these are rusted you can be assured that rust damage continues into the floor area where rainwater may accumulate.

In the Summer your A/C system may leak condensation from the evaporator onto the driver side floors if the evap drain tube is blocked. This is not a likely culprit this time of year, but I figured I'd mention it since we are on the subject.

In any event, this is a common area of rust on the 123 coupe & sedan. If you don't get it fixed the seat rails will rust out along with the floors, rear first.