If you find that your car's battery dies in a day or two when you don't drive the car, it could be that you have an electrical draw somewhere that is draining the battery. First, you would want to find out if this is actually what's happening. Start by charging the battery, and, with the positive battery cable connected and the negative battery cable disconnected, and take a volt meter and connect the positive probe of the volt meter to the negative battery cable and hook the negative probe of the volt meter to the negative battery post. With the doors of the car closed and the key in the 'off' position, the volt meter should read zero like that. If it it does indeed show zero like that, then you do not have a draw, and the problem is going to be the battery itself. If the meter does show voltage like that, something is pulling power when it's not supposed to. Fortunately, the list of suspects on a 65/66 Mustang is very short. You have the horns, the headlight switch, the emergency flasher/ cigarette lighter circuit, the ignition switch, and, if you have a convertible with a power top, the top switch. Also, the charging system has a wire that feeds into the hot side of the starter solenoid, so that, when the car is running, everything electrical is running off of the alternator/ generator instead of the battery, and, the ammeter gauge/ charge indicator light has a wire attached to this same post on the starter solenoid.
None of this stuff is on a fuse in the fuse box, except for the emergency flashers, so, don't start pulling fuses one at a time and checking to see if the draw went away. Unless the problem is with the emergency flashers/ cigarette lighter circuit, which it almost never is, the problem won't go away like that. Start by unplugging the alternator harness from the headlight harness and see if the draw went away. It's the three-prong connector down near the frame rail, kind of underneath where the battery is. If the draw went away, plug that back in and unplug the connector from the side of the voltage regulator. If the draw reappears, the problem is the alternator. Check and see if the black wire with the yellow stripe attached to the 'bat' post on the back of the alternator is shorting itself out on the alternator case somehow and, if yes, fix that. If it isn't, then you need a new alternator. If the draw does not reappear with the alternator harness plugged in and the voltage regulator unplugged, you need a new voltage regulator.
If the draw is still there when you unplug the alternator harness, then the problem is with one of the other systems mentioned, and, you will probably need to remove the instrument cluster to have a look back there. If you look in the april of 08 section here, there's a post on how to do that. It isn't difficult at all. Before you do that, though, go ahead and pull the emergency flasher fuse and see if the draw goes away. If no, go ahead and snatch the instrument cluster out. If yes, put the fuse back in and unplug the wire from the back of the cigarette lighter and see if the draw goes away. If yes, replace the lighter.
Once you have the instrument cluster out, disconnect the headlight switch connector from the headlight switch and check to see if the draw went away. If no, the problem is not with the lights or the horns, since the horn circuit gets it's power from the headlight switch. That leaves the ignition switch. You can check the wire that is hooked to the + or 'bat' post on the coil with the volt meter to see if it shows power with the key in the off position, but, if it does, you need to replace the ignition switch and you have to remove the instrument cluster to do that, so, it wasn't needless work removing the instrument cluster.
If the problem went away when you unplugged the headlight switch connector, plug the connector back in, and unplug the turn signal switch at the base of the steering column. If the draw goes away, the problem is something in the turn signal switch, most likely the horns. If the draw does not go away, unplug the connector behind the dash, kind of up above the fuse box , that the taillight harness plugs into. If the draw does not go away, the problem pretty has to be up front with the headlights or front parking lights/ turn signal lights. It isn't complicated, it's just a question of picking a place to start and working your way through until you find the problem. You don't want to fall into the trap that ensnares many, which is assuming that the problem is with component A and replacing it, only to discover that the problem is still there, so you then replace component B, C, D, etc... Don't start just throwing new parts at the car until the problem disappears. Take a minute to figure out what's actually wrong, and then fix that. You'll be much happier with this way of doing things in the long run.
