I think I have found the culprit of my Volvo P1800 water leak during rainy day. It is from the air vent of the hood.
Previous owner has sealed the air vent with metal plate, and the vent plate is tighten to the metal plate via 4 screws. Recently heavy rain has caused my car got wet inside. I checked the leaked water flow and found screw from inside the dash underneath to be the source of water flow. It leads to the air vent outside the car.
I could lift open the air vent partly, and it exposed the hole beneath the screw and that’s the cause of water going into the car. The nut that was supposed to use to tighten the screw was missing, and the silicone rubber was not fully sealed.
I couldn’t get to access the screw underneath my dashboard. I can see the screw but couldn’t reach it with my big hands. I guess I will need a flexible screw driver + nut driver to lock the screw and nut back into place, and of course put some windscreen rubber seal before that.
Not going to DIY as I do not have enough tools for the work. I will ask my local mechanic if he can do that or not.
I am glad to have found the culprit. Hopefully that solve the water leak issue.