1987 240, my two taillight break lights do not work how do I figure out which side of the light bulb holder is ground thanks

 

One way you can find out is by using a voltmeter on the ohms setting and put one probe on a bare metal part on the cars body and the other on the circuit board contact that the bulb plugs into, when you have a reading that will the ground side .

 

the ground is almost always the outside of the bulb base, and power is applied to the pin or pins on the bottom of the bulb.