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Check the cowl drains at the bottom rear of the fenders. Are you able to easily break/crumble the existing seam sealer? If it's crumbly then you have alot of work to remove/clean. Another trick is a piece of 5/16-3/8 clear tubing on a caulk gun, use a self leveling urethane like osi quad so you dont make dams on the horizontals. Any resealing, you must be sure of a bond with new sealant, A bead on top may not seal,has to be stuck to whatever base you have so "stir" as you apply. An rv supply also has good urethanes that are easier to apply and flow nice.
Sometimes leaks can be found by blowing high pressure, high volume air (shop vac or leaf blower) into the vehicle interior while using a soapy solution that can be spread around areas of possible leaks. Bubbles may show up at the location of water leakage. Hi volume air is needed because the HVAC external air source and the external air vents will leak out the air being introduced.