The wiring diagrams for the later models can be fairly intimidating, but one of my favorite references is the MAD article:
http://www.madelectrical.com/electricaltech/amp-gauges.shtml
Scroll down the page to the simplified diagram, which really outlines the idea of how these girls were laid out.
FIRST problem that is typical of these old girls is exactly what the article is about -- corrosion and heat damage to the bulkhead (firewall) connector which causes all sorts of problems. This diagram ONLY shows the two main leads -- the RED power fed coming into the interior to the ammeter, and the BLACK charging lead coming from the alternator into the interior, to the ammeter.
NOTICE the "welded splice". This is a big, welded splice, taped up in the harness under the dash, a few inches down from the ammeter black wire. As you can see, this feeds the fuse panel "hot" buss, the main feed to the ignition switch, headlight (only) power to the headlight switch, and a couple of other things on some vehicles.
NOT shown is the "ignition run" wire coming off the ignition switch, which feeds power to the instrument cluster, warning lamps, and branches off, goes out through the bulkhead and feeds several things depending on the car model.
This dark blue "ignition run" or IGN 1 feeds the
ignition system
alternator blue field wire
regulator ignition terminal (!!!!voltage sense!!!!)
electric choke, if used
distributor retard solenoid if used
idle solenoid if used
some smog doo dads on some cars
WHY is this all important?
Because if the bulkhead terminals are poor, or the ammeter is AFU, corroded, loose, etc, and or if the IGN run feed is suffering voltage drop
THIS CAN CAUSE an overcharge condition
You may have several problems
First thing I'd do is try to find out if the charging voltage is too high, and WHY
Try to get the battery "up and normal" and run the engine to simulate "low to medium" cruise. Use a good digital meter, and measure the battery voltage. With the engine/ regulator warm, voltage should be somewhere around 13.8--14.2. In no case should the voltage be below 13.5, or higher than 14.5
If so, more checks are in order, post back.
For your fuse, there may be:
A loose/ corroded condition right at the fuseholder, causing the fuse to heat up and blow
A short on something on that circuit, causing the fuse to blow
It might be that if the charging voltage is too high, the lamps are drawing so much current from the high voltage, that the fuse is blowing. Usually, though, the bulbs would burn out!!!!
According to my book, the cig lighter and turn signals are not on the same circuit
According to what I show, "cavity eight" supplies
backup lamps
radio
turn signals
reverse gear indicator lamp
