Wet weather, wet interior???

[chewed 71'er]

Wet weather in the mid-west, noticed leaks in cab of my 71 C10. Under the dash, in the upper left and right corners, rain water is seeping in around seam sealant. What is the best way to access these areas, from underneath or topside? What is typically used for resealing? Also, the inner door panels are rusted as well. Do the exterior vent window seals leak (letting water run down inside the door?), or do the doors leak elsewhere? A little wet on the inside and not liking it!