1987 Mercury Sable rough idle and a misfire also a electrical short

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1987 MERCURY SABLE
4 CYL
FWD
AUTOMATIC
93,000 MILES

1987 Mercury Sable mileage: 93,000. I have two major problems at this time. I have a short somewhere in the vehicle and my mechanic cannot find. The battery and alternator were both changed and there is still something shorting out to the point that I have to disconnect the battery every night or the battery will die. I was told to disconnect the alarm system, which I did but it is still happening. I have checked the following to see if it was drawing power; the trunk the hood, glove compartment, etc. When we connect the positive battery it sparks as if something is drawing power, though we know that the clock draws little power because it stays on. I also have a rough idle and a miss in the engine. The engine was compression checked, the fuel injectors were checked and are working well. The mechanic says that it may be the computer because they could not find the miss in the engine and/or rough idling. It feels like there is a cylinder out. The vehicle was given a complete tune up and problems still persist. Please give me your professional advice. Desperately seeking help.

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Here is the procedure we use to isolate a battery drain. Remove negative battery cable from the battery. Using a 12-volt test light, hook one end to the negative battery post the other end to the negative battery cable you just disconnected. The test light will glow or "light" if there is a drain. If the "light or glow" is faint, that is probably normal draw for the clock or computer. If the "light or glow" is bright, then there is a large drain. That should be corrected. Now start removing and replacing the fuses one by one until the light goes out; that one will be the circuit with the drain. Remember to hold in the button in the door jam for the interior lights. Now about the "miss" in the engine. We would suggest doing a cylinder leak down test, also check for a "flat" cam. If the problem still exists, scan the PCM (computer) for trouble codes.