That valve is under the car, forward of the gas tank, kinda below the drivers-side back seat. Not too hard to change, but there's not a lot of room to work. You'll need stubby or angled screwdrivers. I also helps if your fingers are really skinny and can bend every different way.

Got jack stands? I wouldn't go messing around under there with the car supported on a jack.

Helm manual is better than Haynes, but of course it costs more...

About the brake light... Is it the warning light for burned-out brake light bulbs in your taillights? Or the warning light for the brake fluid reservoir? I think they're different warnings on that car. Brake fluid warning might be shared with handbrake warning, if you're looking for shorted wires.