I sometimes get confused between things I’ve done on the XJ40 I used to own versus the XJS I currently own. An obvious source of water leaks is the air vent cavity on the cowl outside the center base of the windscreen (houses the W/S wiper motor).
In my '89 XJ40 (I think) that air vent cavity/plenum had nipples in the firewall at each end/side that routed rainwater out through the firewall & forward several feet, via rubber fittings on long straight metal tubes (about 3/4" dia) that sloped gently forward to rubber fittings & nipples on each of the two inner fenders, dumping the rainwater out into the wheel wells. When those drain tubes were blocked by decayed leaves, the water level in the cavity rose and spilled into the air passages and I got wet feet. The air passages also have water drains (going to tubes near the transmission, I think), but those can only handle minor water leaks. I know I cleaned out those leaf-plugged tubes several times with water pressure from a garden hose (but, again, I suspect that may have been on my XJ40… its dark & raining presently so I’m not gonna go look at my XJS right now) …anyone know where the equivalent wiper cavity drains are in the XJS?
Also, check these prior posts:
[xj-s] It STILL leaks water!
[xj-s] Wet from the wiper box and "leak-map" for XJ-S
[xj-s] Leaks Into Footwells
[xj-s] Water Leaks Into Footwells
[xj-s] Water in drivers footwell information for all
[xj-s] 2 Hoses?
[xj-s] rain leak