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> After a recent downpour of rain while my car was parked, I found that
> the floor area behind the driver's seat was saturated!
Ummm, were your windows closed?
Actually, it's probably the sun-roof tray overflowing and running down the side pillars. Depending on how you're parked, the front or back carpets may end up wet. Contrary to what you might think, the sun-roof does not make a watertight seal, but rather sits over a collector tray. In each of the four corners of the tray is a drain hole (about 1/2" ID) with a drain tube that goes down the corner pillar. With a small light and a lot of neck gyration you should be able to see these drain holes. You should also be able to spot where the drain tubes come out underneath the car.
Unless you never open it, the sun-roof tray will easily collect debris. The drain tubes need to be kept clear in order to prevent the sun-roof tray from overflowing. This is part of routine 740 maintenance. Additionally, if the sunroof felt edge strip becomes badly worn or if the roof panel gets out of alignment or doesn't fully close then more water can enter the tray than can be handled by the drain tubes. This is especially a problem if you're parked on any kind of slope during a downpour since one or two tubes have to carry the whole load.
Keeping the drain tubes clear can be as simple as blowing a good shot of compressed air up each drain tube. Make sure that undercoating isn't obstructing the ends of the drain tube. I also like to attach a length of clear 1/2" tubing to a vacuum cleaner nozzle and probe the corners of the sun-roof tray to remove any debris, but blowing the debris out would do the job if you don't mind cleaning up your interior. To remove stuck debris like leaves you can try fishing with a coat hanger, perhaps with a bit of sticky tape wrapped on the end. If a wet leaf ever gets jammed down a drain tube you may need to ream it out using a long piece of stiff wire. Since you can't see down the tubes it's probably a good idea to ream them out if you're having any drain problems.
There was also a Tech Service Bulletin out on late 80's 740 sun-roof trays to install a raised moulding around the tray edge. Your '89 may already have it. As I recall it required a fair bit of effort to install, so I'd only pursue that option as a last resort.
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