Battery keeps draining?

An electrician friend told me about the vehicle listed above that he and others were battling with for a long time but to no avail. The major problem is that the car battery keeps draining if it's not disconnected. On the dashboard the check engine light remains on but fainted/dim even when though the car is shut-off. But it's bright alongside others whenever the ignition switch is turned on to position. How can I fix the problem?