{Please see the FAQ} - AWJ
The above link will take you to some good discussions on the subject. In summary, if your lens insides are not dirty, you can just dry out the lights and reseal the leaky top seam. That seems to be where the water gets in.
I just did it to one of mine after the first rain I experienced in my new 92 SC300. I removed the light housing, placed it "holes down" over a floor air register in my house for a day to dry it out, then sealed the top seam above the parking light side. There is a low spot there in the seam and mine showed cracks in the sealant. You are sealing the seam where the black plastic rear housing meets the lens assembly. I used hardware store exterior 100% silicone "door & window" caulk, because I had some in the garage. It seems to have worked.
Although it only appeared to be condensation, there was actually an ounce or more of rainwater inside the housing. There is debate about whether or not to drill a 1/8" hole into the lens under the lower corners. Personally it looks easy and could be effective. Several people that have done it swear by it. Others that have not done it tell you not to. I guess I believe those that have done it versus those that have not. But I did not do it as it does not seem to be needed yet in my case.
If you let it go, eventually your lenses get dirty inside. If that is the case, you can read the threads and find the procedure for warming the assembly in an oven for a few minutes, which looses the factory sealant goop. Then you pry the black housing off, clean things up, then re-seal and reassemble. I have no experience with that.
Hope this answers your question.
:D
-Don M