Do the headlights dim when you turn the key to the crank position?
If the headlights stay bright, but the radio, heater and turn signals don't work, it's either the bulkhead connector, fusible link or ignition switch connections.
If the headlights stay bright and the radio, heater and signals work it's not the fusible link. Check cranking voltage at the "S" terminal on the starter. If you get 7 volts or more, it's the starter. If 7 volts or less, check cranking voltage at the ignition switch solenoid terminal. If you get 7 volts or more, replace the purple wire from the ignition switch. 7 volts or less, replace the ignition switch.
If they dim, check the voltage at the battery while cranking (key in "Start"). If you have less than 9.6 volts, test the battery. If the battery is OK, repair the starter. If you have 9.6 volts or more, put the (+) lead of the meter on the engine block and the (-) lead of the meter on the (-) battery terminal. If you get .5 volts or more with the key in the crank position, clean/tighten or replace ground cable. If you get .5 volts or less, check the cranking voltage at the "B" terminal of the starter. That's the one that the (+) battery cable is connected to. If you get 9 volts or less, clean/tighten or replace the (+) battery cable. If you get 9 volts or more, check the bulkhead connector and fusible link.
The fusible links are the 2 small wires with the black plastic barrel shaped thing on them coming from the "B" terminal on the starter. One goes to the alternator and the other one to the dash connector at the ignition switch. They're both red. Check them individually by using "Ohms" or continuity on the meter. Disconnect the (+) battery cable from the battery. On "Ohms" you should read near 0 or 0 if it's good. On continuity the meter should beep if it's good.
Tommy