I have heard that taillight can leak on 90’s civics and I just check and discovered moisture and a small drip after I washed my car. I also noticed some yellow filler that cover part of the light. Maybe some previous owner tried to fix it? There is not rust also but I do hope maybe I can prevent it, ideas?

 

If you could add a picture it might be easier to tell what the problem is.

Taillights usually leak when the seal between the lamp and the bodywork is damaged.