The tailight is always (and only) energized through the brown wire, through the fuse, from the ignition switch. When the ignition key is in the 'Park' position, battery voltage is fed from the red wire through the ignition switch to the brown wire. When the ignition key is in the 'On' position, battery/alternator voltage is fed from the brown/white wire through the ignition switch to the brown wire. The brown/white wire also energizes the instrument lights, so if they are not working either, then the problem is that your brown/white wire isn't getting voltage.
On earlier models with an 'On-Off' switch for the lights, the brown/white wire is energized when the ignition key is 'On' and the light switch is 'On'. On later models without an 'On-Off', the brown/white wire is energized when the ignition key is 'On'. I believe 1976 was the last model year that had an 'On-Off' switch for the lights, and 1977 was the first model year without it, so depending on the date of manufacture, yours may or may not have one. If it does, the black wire at the 'On-Off' switch should be energized when the ingition key is 'On'. If the black wire has no voltage, check for loose wiring connections behind the headlight (inside the housing).
The brown/white wire also provides voltage to the turn signal switch, so check to see if they are working. If so, then check for loose brown/white wiring connections behind the headlight (inside the housing). If the turn signals are also not working, check the wiring connections at the ignition switch and its harness connector.
