Thanks for the confirmation. I've been working on instrument clusters for a couple weeks. The circuit for the Brake Wear indicator is on one of my spares so I became curious about the relay. The battery light receives a ground at pin 8 of the triple gauge connector from 61/D+ at the voltage regulator when the alternator is not spinning. When the alternator is spinning, 61/D+ presents regulated 13+ volts at pin 8. The other terminal of the battery light bulb receives circuit 15 (start / run bus) power at pin 7 when the ignition is in key position 1 or 2. Voltage at both terminals is a groundless loop so the battery light extinguishes. The brake wear indicator taps pin 8 internally for the ground feature when the alternator is not spinning. The relay on the back of your triple gauge is in series with a diode and resistor. This circuit blocks regulated voltage to the brake wear indicator when the alternator is spinning - open circuit.
There is a spade terminal (seen behind the relay in post #1) that is the output of a grounded state of pin 8. It is joined by the brake wear sensor leads. When any of these leads presents ground to that spade, the brake wear indicator is grounded. When start / run bus power is present at the + terminal of the tachometer (protected by fuse 10 in 1979 ETM), the brake wear indicator bulb gets power. When both conditions are present, the bulb illuminates.