Chris, my bulb check unit is located in the relay box behind the main fuse panel (left side by firewall). You remove the fuse panel lid and loosen six screws and the relay lid comes off with the fuse panel top frame. The bulb check unit is the long relay on the left. If you have a good wiring diagram for your model, you can remove the relay and check the wiring continuity back to the lamps.

However, most problems are going to be poor connections in the rear lamp unit itself, e.g., poor ground connection, lamp socket connections, cold solder joints, deteriorating plastic, etc.

Brent Smith
Snellville, GA
1986 300E 294,000 miles