Mitsubishi Repair: Battery Drain, mitsubishi eclipse, fusible links

Question
1993 Mitsubishi Eclipse 1.8L, Auto. I am trying to trace an electrical problem. The battery is draining whenever the car sits overnight. It has a brand new alternator. Also at one point cables for jumpstarting were hooked up backwards. It blew the alternator fusable link and I had to replace the alternator. I got the car this way it now runs and charges but won't hold a charge. I also had the battery tested and it showed good.
Any suggestions would be helpful.

Answer
Charles,
There are many bad things that can happen if a battery is installed backwards, or if the car is jumpstarted backwards.  Actually, jumpingstarting it backwards can be worse since its a larger surge of current on the system.

When a battery is jumpstarted backwards the electricity was let loose and was possibliy burning through wires, your alternator, computer and other fun stuff.  Your fusible link stopped that excessive current flow, but the damage may have already been done.  If you can, install your battery in another car, and see if it holds a charge while in the other car.  If the battery holds a charge in the other car, then that means your car has a short somewhere in the charging system.  You'll want to check all fusible links, cables, and alternator connections again.  You may have burned through a wire, and it's causing your system to discharge.  While the car is running, your alternator should be producing a voltage of around 14.5 volts to the battery. If you have a voltmeter, you can check the voltage throughout your system to find where the drain is.

But if your battery does not hold a charge in another car, that means your battery still has a problem.  Have it checked again.
Good luck!