The red BRAKE light on my dash is on while I'm driving. The Emergency Brake is off. The manual just states to investigate. Any idea what that is for? It is a '98/4 cyl

 

I just had a similar incident...some light bulb is out...mine was the second license plate light...check all your rear lights.

 

Brake light may also be connected to the brake fluid reservoir. The reservoir cap has a float connected to a switch, check the brake fluid level and float.

 

I've have the same problem on my car.

Emergency brake switch is fine.
Reservoir level and sensor is fine. (After i change it)

All tail light's is fine, What's next??

 

98 Camry: filled up power steering reservoir and verified two white lights above license plate are functioning. Reverse lights work as expected.

What is the next diagnostic step?

 

Make sure if equipped the brake fluid level sensor is connected electrically and mechanically works. If equipped make sure the brake fluid level is correct.

The sensor ONLY monitors the lower brake lights, NOT the upper light in the rear window or anything other light bulbs. The sensor monitors electrical bulb circuit resistance.

Suggest you replace the lower brake light bulbs. When doing so check the bulb socket electrical terminals for corrosion and or just clean them for good contact.

Its possible the sensor device has failed which happens but check all other possible issue first..