Rain water seems to be trapped underneath my car somewhere. Also, The brake light on my dash is on, although all of my brake lights work perfectly.
Hi, first off is there any more information about the water you can give us? The car has drainage areas for water to flow through which can get clogged up with debris and leaves. Water will build up and can then leak inside the car. This should be checked out soon as there are electrical components that can get damaged as a result.
As for your other issue, typically the brake light on the dash is warning you that the brake fluid is getting low in the master cylinder. As your brake pads get thinner, more fluid is required in the lines to push them against the rotors. You will see the level in the master cylinder slowly drop as you drive the vehicle and the pads get thinner. Eventually the fluid sensor in the master cylinder will trip because the fluid has dropped too low. A certified technician, such as one from YourMechanic, would be able to inspect the drainage areas and check your brake fluid level for leaks.