I have a 1999 buick century that is leaking coolant on the passenger side just under the coolant tank

SOURCE: how to replace a coolant level sensor, 2000 buick century

The coolant level sensor is located on the radiator. If you are looking at the engine from the front of the car it will be on the left side. There will some kind of clip holding it in the radiator. Removed the clip and pull the sensor out. You will lose coolant so try to swap it out quick or drain some of the coolant out. Once new sensor is installed, install clip to keep the sensor in place and reconnect the wires to the sensor. Crank the engine and make sure the coolant level is full. This should correct your problem.

SOURCE: antifreeze/coolant leaking from under passenger side dash

If it's your heater core, unless you're a mechanic you probably should get professional help. The heater core is located on the firewall and many parts need to be replaced to get to it. Do you see water on the floor inside the vehicle, Many times a heater core will leak inside.

SOURCE: coolant leak 1998 buick external

it sounds like the water pump may be leaking beacuse when runnung the seal is lubricated and may not be leaking at that time but when left over night the seal in the water pump contract and starts to leak. check for stains and leaks with a mirror before starting in the morning i think you will find that it is leaking. don't worry they are not bad to change. good luck

SOURCE: 1998 Buick Century Driver-side blows hot, passenger side cold

vent your vent port one your port may be block or you temperature sensor giving trouble

SOURCE: How do I replace broken 2002 buick century passenger side mirror?

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. Then you can access the bolts that secure the mirror onto the door