The two issues may actually be related. 1. Battery keeps dyingIf you have water in the driver's rear footwell, I'd be suspicious of water intrusion affecting wiring or electronic modules before assuming the new Interstate battery is bad. Common causes on a 1998-2000 LS400 include: Parasitic battery drain from a circuit staying awake Alternator diode failure (alternator charges normally but drains the battery when parked) Trunk light, vanity light, glovebox light, or door switch staying on Water intrusion causing corrosion in wiring harnesses or connectors Aftermarket alarm, stereo, GPS, etc. A quick test: Fully charge the battery. Disconnect the negative cable and connect an ammeter in series. Let the car sit 15-30 minutes so modules go to sleep. Current draw should generally be under about 50 mA. If it's higher, pull fuses one at a time until the draw drops. Because you have known water leaks, I'd inspect: Driver-side kick panel connectors Left rear floor harnesses under the carpet Any wiring beneath the wet carpet Wet carpets can hold water for weeks and create electrical problems. 2. Rear sunroof drainsThe LS400 sunroof cassette has four drain tubes, one at each corner. The fronts usually exit behind the front fenders and are easy to access. The rear drains are harder to find because they run down the C-pillars. For your 1999 LS400: Open the sunroof fully. Look into the sunroof tray. The rear drain openings are located in the rear left and rear right corners of the sunroof cassette. They are small holes tucked into the corners and can be difficult to see without a flashlight. To clear them: Use a flashlight and find the rear corner holes. Feed flexible weed-whacker line (not metal wire) into the hole. Gently work the line downward. Pour a small amount of water into the rear corner and verify it exits underneath the car near the rear wheel/C-pillar area. Do not use compressed air at high pressure because it can blow the drain tube off the cassette. One thing to verifySince you found water in both the front driver footwell and rear driver footwell, I'd also inspect: Driver-side windshield molding/seal Cowl drains under the windshield Sunroof drains Door vapor barrier behind the driver door panel A clogged rear sunroof drain would definitely explain water appearing in the rear driver's side floor, but water in the front and rear driver's side sometimes points to a cowl drain or windshield leak as well.