You will have to play detective. I have done this a couple of times. You have to be methodical and it is better done in the summer in my opinion but it ain't summer!! What I do is dry out everything as much as possible. This can be quite a chore if it is really wet. Next, try to park the thing where it is covered or use a tarp so it does not get worse before you fix it. Then use the garden hose test to apply water to different areas and see if you can detect where it is leaking. (Don't spray at the car, just simulate rain, or just let the hose pour jently on the car) Get a bright flashlight. Have someone else use the hose while you are inside the car if possible. (Do not get in the trunk!!) It helps a lot if you have a good guess about the general area where it is leaking but water can move around so it can be tricky. I have found leaks at lamp assemblies as I mentioned and also at body seams (Subaru). Also areas around windows that are rusty. Water can leak just about anywhere there is a pinhole or bigger. It will be hard work to find but the payoff is really good - a dry car!!

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra