lights on warning buzzer & battery buildup

My wife has a 94 Toyota Tercel "school car". These didn't come equipped but I would like to install a lights on warning buzzer. I can't find any wiring diagrams. I know you hook the buzzer between the door light and the headlight switch. I can't find a box where both circuits are located.
Does anyone have any idea where to look and how to hook these up?

Also, I have a buildup on the battery posts and hold down bracket. I have cleaned and changed the connectors but the build up still exists at the connections and the hold down bracket. I presently use baking soda to neutralize the acid. What could be wrong?

Try coating the battery terminals with dielectric grease.

For the buzzer,

Go to radio shack and get a buzzer. Use the marker/tail lights' wiring for a power supply and a door jamb switch (the switch in the jamb that completes the circuit to turn on the interior lights can be spliced into) to connect to ground. When the door opens the circuit is completed and the buzzer sounds if the lights are on.

another option,

You can install a relay into the lighting circuit so the lights can come on only if the car's ignition switch is in the accessory and on position. I don't have a diagram of the lighting circuit so I can't tell you what wires to splice the relay into. Bosch relay, connectors, and a length of wire should cost less than $10.
The way to wire the relay. Use pins #30 and #87 to splice the power wire to the headlight switch. Connect pin #85 to a power source that is live only when the ignition switch is in the accessory and on position (power source for the cigarette lighter maybe an option, tap into the unfused side of the circuit so if the fuse blows the headlights will still work). Ideally, look at a wiring diagram to find the appropriate wire from the ignition switch. Connect pin #86 to ground. With this set up, you can't turn on the lights without having the key in the ignition.