I would use the DC Volts setting to trace the voltage starting from the wire going to the tail light assembly and ending at the lamp socket. The black wire should be connected to chassis ground and the red wire to measure the 12V DC volts.

You could also get a circuit tester (from an autoparts or hardware store) that has a built in light instead of using a multimeter.

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles