After many years of a soggy wet passenger floor board, I found the fix for my 90 LS. As mentioned in Teg tips there are some gaskets for the plastic push pins on the secondary cowling–short black one on passenger side, that was not my problem.
The water was leaking from the center of the the main cowling (it has a fresh air intake sort of grate from the middle over to the drivers side) running straight back along the cowling and into the four square air intake cutouts on the firewall. The water ran along the black plastic a/c duct and dripped off of the passenger knee bolster.
The FIX: remove main cowling, turn it over and remove the angled plastic piece that is supposed !!to divert water away from the firewall cutouts. I took electrical tape and covered the cowling grate almost over to the drivers wiper hole. Then apply silicone over the tape. I then turned the cowling over and filled in the grate with silicone over my patch.
No more leak…now its on to the trunk…thanks for the tailight gasket tips…