OK, so I've been thinking about this. The 8 pin connector for the 78 Van is as follows:
1) 18 G white - power from the brake light switch
2) 18 g dark green - to the right rear turn
3) 18 g yellow w/ black line -to left rear turn
4) 18 G red - power from turn signal flasher
5) 18 g dark blue - power from hazard flasher
6) 18 G light green - right front turn
7) 18 g yellow - left front turn
8) 18 g dark green with white line - power from the horn fuse

From this list, you should have power coming into the turn signal switch from the following wires:
Turn flasher with the key on.
Hazard (4 way) flasher any time.
Brake light switch only when the pedal is stepped on.
Horn any time. The horn honks when the power wire is grounded.

Generally, wires from the turn switch that have yellow on them are for the left turn (front or rear) and wires with green are right turn (front or rear, except the horn wire)

From each of the front turn wires there may also be a wire for each side of the same color that is a much smaller wire, those would be for the turn signal lights on the dash. On the newer stuff (like this 78) those wires would have been pulled off the body side harness, they may be the other 2 wires on your connector. At this point, I really wouldn't worry too much about any extra wires in the connector. It would simply mean a secondary circuit doesn't work.

If you power these wires, your turn and brake lights should work. You should be able to use a jumper wire from a hot source to each light wire and see that light light up on your car. Gene