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The leak in front is most likely that the windshield needs to be resealed. The leak in the rear is probably the weatherstrip around the door not sealing correctly, maybe it's twisted, or not fully in the channel across the top.

As for the windshield, the only place it can leak in front is where the glass seals to the body. If it's leaking water, the seal is bad. The only way to repair it is to remove the glass and reseal it. Very tricky to remove the glass without breaking it, best leave that to a professional. Adding sealer without removing the glass is NOT the way to do it.

The high mount stop lamp can leak, the roof rack can leak, and the weatherstrip can leak. Climb inside and have an assistant spray water over while you determine where it's coming from. Leaks in the rear of the vehicle cannot travel to the front, (windshield area).

If it is on the pass. side, it is more than likely the heater core. Replace. Should take about 2 hours and can be done with out removing the blower motor form the pass. side footwell.

I had wet carpet in rear of 2000 ASTRO caused by a plugged rear AC condensate hose. The hose is located under left rear fender behind left rear wheel. I used a piece of clothes hanger wire to unplug hose. There is another drain hose for the front unit located under the right front fender & behind the wheel.

have you checked the heater core? my suburban was soaked in front and rear before i even noticed that the radiator was bone dry we did a pressure test