This is assuming your car is a 240 sedan, right?

Of all the excellent engineering and design things on a 240, the ain't one of them. Water runoff from the top and rear glass flows into the trunk lid seam, goes down and - splat - hits the seal between the lens and the tail light housing.

The sealant is not eternal so it gives way and you get a leak into that light. Leaks are always the lens sealant failing. Only the clear back-up light lenses are removable. You might be able to carefully remove and reseal that one.

The other lenses have to be drained. Use a 1/16 or 1mm drill and make a hole in the two lower corners of the leaky lens.

Water will eventually get into the trunk. Feel along the bottom of the inside tail light cover. Fold up some newspaper and put it along there - look for wet spots. Feel under the carpet to see if the under padding is wet. If it is, remove it ASAP. Replace with 10 to 15 layers of old newspapers which will at least absorb the water and allow you to throw it out. BTDT

Good Luck,

Bob

:>)