1994 GMC Sonoma water leaking inside

First I would try to narrow down exactly or close to where the water is coming from.
By pulling back the carpet and spraying water in parts of the grill in front of the windshield and then around the windshield.

Chaces are it's a seam seal in the vent box where the wiper arms are and where the fresh air is drawn into the heat/ac system.
Seams can be reseald using anything. I prefere to use a rtv sealant but you can buy seam sealant at auto paint supply stores.

If it's a windshield seal that will require it to be removed to be properly sealed.