Where is the diagnostic plug exactly located in my mercury sable 1992?

Your diagnostic plug is located under the hood, driver side, front side of strut tower, mounted to fenderwell. I hope this helps and good luck.

Under hood, driver side, front side of strut tower, mounted to fenderwell

Note, this is an OBD1 connector. You will need an OBD1 scanner to read codes.

Alternate methods to extract trouble codes:

USING THE ANALOG VOLTMETER See Figures 3 and 4
Service codes will be represented by pulsing or sweeping movements of the voltmeter's needle across the dial face of the voltmeter. Therefore, a single digit number of 3 will be reported by 3 needle sweeps. However, a service code is represented by a 2-digit or 3-digit number, such as 23. As a result, the Self-Test service code of 23 will appear on the voltmeter as 2 needle sweeps, then after a 2 second pause, the needle will sweep 3 times.
Connect the analog voltmeter as follows:

1. Turn the ignition OFF .

Fig. Fig. 3: Voltmeter hook-up

Fig. Fig. 4: Analog voltmeter output code format

1. Set the voltmeter on a DC voltage range to read from 0-20 volts.
2. On all EEC-IV vehicles, connect the voltmeter from the battery positive post to the Self-Test Output (STO) pin of the large Self-Test connector. On all other vehicles, connect the positive voltmeter lead to the EEC STO line and the negative lead to engine ground, then jumper the EEC Self-Test Input (STI) to ground.

USING THE MALFUNCTION INDICATOR LIGHT (MIL) During the Self-Test,a service code is reported by the malfunction indicator light. It will represent itself as a flash on the CHECK ENGINE or SERVICE ENGINE SOON light on the dash panel. A single digit number of 3 will be reported by 3 flashes. However, a service code is represented by a 2-digit or 3-digit number, such as 23. As a result, the Self-Test service code of 23 will appear on the MIL light as 2 flashes, then, after a 2 second pause, the light will flash 3 times.