Water leak when it rains

Leaks can be very deceptive and difficult to track. Thankfully this is a truck so there is a smaller area to work with. First thing would be to look at where the water collects. Say it's the drivers side footwell, you would start on that side. I tend to start by using a good light and work my way around the area looking for washed areas or water marks. Using the footwell example I would look at the door opening, any marks where water passed? Then under the dash to look at the A pillar area and up under the dash at the firewall and cowl area. Any marks or stains? Then look at the area around the windshield, are there any rusted areas, is the glass still bonded to the cab? Check the rear window as well. There are a few common locations for leaks on vehicles, especially older ones, The glass is essentially glued in, rust or paint issues can cause the bond to fail and water can get in. Then the body seam where the firewall, cowl panel and pillars all join up, there are a lot of seams and the seam sealer can fail. The HVAC has opening under the cowl for air into the body, they can leak. There is a drain line from the HVAC case to handle some water and any condensation that comes from the cores, they can get plugged. Door seals can get torn or pulled free. The rear glass and the third brake light seals can leak. You know it only leaks in the rain, so ask yourself a few questions, Does it have to be a hard rain? Does it matter if the truck is moving or stationary? Where does it get the wettest? Then for testing the quickest way for a DIY is a water bottle or watering can. First look for signs and stains. Next look above those for seams or parts that can be loose. Not really seeing anything? Time for the next step apply water to one section of the vehicle, like the center to left side of the windshield. Any water inside? No, apply water to the other side. Still nothing, go around each door and window on the drivers side. Keep going to the rear glass, CHMSL and then to the other side. It might take a few times around to check everything. Another way to find leaks is to use a smoke machine, for that you use the fan inside the truck, turn it on high with outside air, then you move the smoke machines nozzle around the different areas and look for the smoke to blow away from the area of the leak. This is handy for glass and seam leaks but can be difficult if you don't have good access. If the truck needs to be in motion it gets trickier as those could be coming from water spray against things that don't get wet under falling rain. That is when you have to look at the firewall, up under the cab at drains, the floor panels themselves and the seams around those panels. It can take a while to find them, and depending on the location the repairs can be difficult.

The truck has been parked and I just bought it. I have marked the spots that I have seen wet runs while it's raining. I'll take a video and try to upload it

Are the spots all in one area or are they spread out? Will wait for the video, but if they are spread out I would start with the glass, gently push on it and see if it flexes from the body of the truck. Also look at the cab mounted light(s) they only seal with a thin foam that degrades. For testing a quick thing is to use painters tape and apply it around the area to stop water, then dump water over it, still have leaks? Likely not the places you applied the tape.