What is causing the battery to drain?

Running the car will drain the battery it if it's not charging ! An if your charging light is on it isn't charging ! Did you check power an grounds on the alternator ? There is a single heavier wire on the back of the alternator, this should have battery voltage ! You may want to take this to a ASE certified repair shop !
Functionality
With the ignition switch in the RUN position, voltage is applied through the warning indicator I circuit 904 (LG/RD) to the voltage regulator. This turns the regulator on, allowing current to flow from battery sense A circuit 35 (OG/LB) to the generator field coil. When the engine is started, the generator begins to generate alternating current (AC) which is internally converted to direct current (DC). This current is then supplied to the vehicle's electrical system through the output (B+) terminal of the generator.
Once the generator begins generating current, a voltage signal is taken from the generator stator and fed back to the regulator S circuit 4 (WH/BK). This voltage feedback signal (typically half the battery voltage) is used to turn off the warning indicator.
With the system functioning normally, the generator output current is determined by the voltage of the A circuit 35 (OG/LB). The A circuit 35 (OG/LB) voltage is compared to a set voltage internal to the regulator, and the regulator controls the generator field current to maintain the correct generator output.
The set voltage will vary with temperature and is typically higher in cold temperatures and lower in warm temperatures. This allows for better battery recharge in the winter and reduces the chance of overcharging in the summer.
Battery Positive Output (B+) Circuit 38 (BK/OG)
The generator output is supplied through the battery positive output (B+) terminal on the back of the generator to the battery and electrical system.
I Circuit 904 (LG/RD)
The I (ignition) circuit 904 (LG/RD) is used to turn on the voltage regulator. This circuit is powered up with the ignition switch in the RUN position. This circuit is also used to turn the charging system warning indicator on if there is a fault in the charging system operation.
A Circuit 35 (OG/LB)
The A (battery sense) circuit 35 (OG/LB) is used to sense battery voltage. This voltage is used by the regulator to determine generator output. This circuit is used to supply current to the generator field (rotor). The amount of current supplied to the rotor will determine generator output.
S Circuit 4 (WH/BK)
The S (stator) circuit 4 (WH/BK) is used to feed back a voltage signal from the generator to the regulator. This voltage is used by the regulator to turn off the charging system warning indicator. The S circuit is fed back externally on external mounted regulator generators.
Visual Inspection Chart Mechanical Electrical

• Battery case, posts, hold-down clamp, cables and connections
• Generator drive (serpentine) belt for condition and tension to make sure there is no slip between the belt and the pulley. For additional information, refer to Section 303-05 .
• Battery charge
• Generator pulley

• Battery junction box (BJB)Mega Fuse
• Battery junction box fuse:
  • 11 (20A)
• Central junction box (CJB) fuse:
  • 30 (30A)
• Circuitry
• Charging system warning indicator
• Cables

1. Check the operation of the charging system warning indicator lamp (instrument cluster). Normal operation is as follows:
   • With the ignition switch OFF, the charging system warning indicator should be OFF.
   • With the ignition switch in RUN and the engine off, the charging system warning indicator light should be on.
   • With the engine running, the charging system warning indicator light should be off.

1. Verify the battery condition. Refer to Section 414-01 .

1. If the customer concern is verified after the initial inspection, refer to the Symptom Chart to determine which tests to carry out.
2. • The charging system warning indicator is on with the engine running (the system voltage does not increase)
   • Circuitry.
   • Voltage regulator.
   • Generator.
   • GO to Pinpoint Test B .
   Your whole problem is the alternator is not charging , a couple tests with a volt meter would tell you !