1999 Lexus ES-300 won't start after sitting for a day or so.

Something is staying on and draining the battery. Check to be sure all the interior lights are off. Make sure any map lights, the blower for the heater and air conditioning, radiator fan, etc. are not running when the ignition is off. Does the vehicle have any aftermarket equipment, like a stereo, that might be staying on? Is there an automatic leveling system that might be running the air compressor periodically to maintain the vehicle height? If you can't find anything a circuit by circuit check needs to be done to find out what the draw on the battery is. This involves hooking up a meter to the battery and removing then replacing each fuse, one by one, until the meter drops. Then the circuit is isolated and the components on the circuit need to be checked. It's tedious but that's the only way to find the problem if it's not something kinda obvious. Hope that helps! Jim