Although there are a number of possibilities, including those others mention, one thing that is well known with 240s is the windshield glass developing leaks around the seal, especially at the sides and lower corners. It can happen with the original windshields, but is even more common with replacement windshields. It may take a few years to develop. Often it first becomes noticeable after a car wash or a bad rain storm.

The seal can eventually fail if the replacement glass wasn't formed perfectly to match the window frame curve or if the glass was installed in less than a perfectly centred orientation, which for 240s seems more critical than many other cars if you talk to the techs.

Another issue is if body rust develops in the window frame behind the seal. This is more common in the 240 windshields older than yours that weren't in a full rubber window gasket. Surface rust would often develop starting near the window clips or any paint areas that may have been nicked or worn through. This often happens out of sight behind the window trim until it gets either so crusty or perforated that water can creep by. Not as likely in your car, but do keep that in mind, especially if you spot rust stains in the paint beginning near the window seal.

One way to try isolating an exterior leak such as yours is to expose the passenger footwell firewall and outer wall area, peeling back the carpets a bit, also pulling the glove box for visibility. Starting from the top, shoot a high pressure hose around the edge of the windshield, also around the wiper arm pivot in the cowling and as well from under the hood shooting around the base of the wiper motor and any other through the firewall gasket area. Have someone inside with a paper towel for blotting, carefully watching with a good light to try spotting any leak under hose pressure.

If you don't have time to do this, an interim step is to try stopping water infiltration by using duct or packing tape all around the right side of the window trim so water can't get behind the trim. If the problem does go away then you'll know that was likely it. If you do identify a window frame leak, a good temporary fix is to shoot silicone sealant behind the window trim. The proper fix is to pull the glass, repair the rusted area and sealing surfaces, then re-install the glass.

Other possibilitiea are blocked sun roof tray drains, worn and poorly sealing rubber door gaskets and, worst of all, a leaking heater core. As mentioned, the windshield wiper assembly has a number of penetrations through the firewall. Be glad yours isn't a 700/900 as a heater core would be at the top of my suspect list for wet carpets in the passenger footwell.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now