Door seals leaking

check rubber door seals for damage/missaligned, look under hood see if gutter run offs are clear of dirt/debri, look for signs of rust body rot.check joints between the wing and door frame.

door not close all the way or door seal is leaking.check for cracks ot tears in door seal

you need to remove the door panel as the bolts holding on the mirror are on the inside. roll down the window.,as the door panel has to come up and out. This is easier done with the window down especially for reinstallation of the door panel. Remove the power window switch assembly ,handle, screws,and the clips that hold on the door panel.you need a flat pry tool for the clips.they pop out of the holes in the door.once the panel is off,the plug for the mirror and the nuts are removed.the new mirror and seal is replaced and the door panel re attached.

There is a great possibilty that there is a leak on the Left Side of the Windshield. It is most likely in the area of the upper left side.
The reason for this conclusion is:
The rain water is coming in the Left Upper Side; and either coming down the "A" Pillar, or traveling underneath the trim on the inside roof and coming down the "B" Pillar.
The only other explanantion is a torn, worn, cracked or dislocated Weather Seal on one or both of the doors, allowing rain water to come in. Inspect very carefully these Weather Seals, especially on the top front portion of the rear door.
To help you locate these after a hard rain:
Take the trim panels off the "A: and "B" Pillars and the Roof Line above both Doors. Check for wetness underneath the trim. Or take these trim panels off, and place folded paper towels under the trim panels; reattach the trim panels and wait until it rains. Afterwhich, take these trim panel off and check the paper towels for wetness.
The Windshield Seal is easily inspected for cracks and tears.
Let me know if this helped, or if you have additional information or questions. Feel free to contact me at FixYa.com!

check seals around the door and the body where door closes, also check where windshield ends and firewall starts, could be small leak around any hoses or lines that go thru firewall, such as heater hoses and screw holes where parts are attached

Some leaks actually are from the windshield and it is contained by the trim so you can't see where it is leaking from. If it is the door seal look at the striker bolt to see if there is any abnormal wear. Make sure the striker bolt is aligned with the door striker. If you don't know how to check this run by your local body shop and ask them to check it for you. Sometimes if the keys have ever been locked in the vehicle people use door shims to hold the door open as to pick the lock. This is bad because it knocks the door out of alignment with the seals. Hope this helps.

I had a similar leak on a 1995 Accord. I traced the issue to the plastic liner behind the door panel. The seal was broken at the bottom and water would leak from that to the rubber weatherstrip at the bottom of the door that seals at the plastic threshold molding. You can't really see it good until the door panel is removed. The water would drip until it puddled up enough to run over the threshold molding and under the driver's seat. This caused the floorboard behind the driver's seat to be soaked after a rain. I have resealed the plastic lining but of course it hasn't rained since. I'm confident this will solve my problem and hopefully provide you a new place to look for yours.

I would check all window and door seals.

Take off side door panel, screws securing it are revealed, unscrew screws remove them and and the mirror drops off. If the wing mirror is on the bodywork, then the screws should be either in the wheel arch or under the bonnet.

On the inside trim, you can remove screws that are by the door handle, arm rest, under door and side of door and then gently pull back on the panel as there will also be retaining clips attached and you don't want to break them. You'll need to lift the door panel upwards in order to get it off the window ledge. Then you'll need to manipulate the panel and disconnect the door handle linkage from the door handle and disconnect the wiring for your controls. You can then disconnect the cable running form the mirror to the control. Then you can access the bolts that secure the mirror onto the door