[oldiron]

With out me having pictures to show you this may be confusing but here it goes.
The two purple wires and the two green wires are for a neutral safety switch.
Connecting the two purple wires was a correct thing to do. Now down on the starter there are three places to connect wires to. The biggest being the battery cable from the battery. The two smaller ones are where a misconnection can be made.
The terminal on the starter solenoid next to the engine block is where the purple wire is to be connected. This is the other end of that connection that you made under the dash.
The other terminal on the starter solenoid that is closes to the frame is a electrical feed by a yellow wire to the points style distributor to feed 12 volts when cranking the engine over. If these two are switched then the truck will not start. By the way if you have an HEI dist. this yellow wire is not needed.

Now for the green wires. These are for the backup lights. There should be a switch on your transmission so when you are in reverse this switch makes the curcuit complete and turns on your backup lights.

For your tail light problem if the bulb is good take a test light and check to see if you are getting power to the terminal. Make sure that the test light is well grounded. If the test light works you might have a bad ground in the light structure. If the test light doesn't work then check the wiring for a loose connection.

Hope this helps.