1. I tried a copper wire test (inserted a wire into the bulb socket and the other end to a ground source), each of the tail lights got brighter.
: Your tail light sockets will have 2 spring loaded contacts - 1 is for the brake/turn signal (the brighter of the lights), and 1 is for the tail lights. I assume you inserted the wire at some point along the side of the socket. If lights got brighter, then most likely it indicates a poor ground.
2. Next I tried a test light. It lit up on each tail light socket with the headlight switch turned off and also with the parking light in the on position.
: With the headlight switch pulled out to its first "notch", your parking lights are supposed to come on. They should also remain on when you pull the headlight switch fully out.
3. There is no power getting to the back up lights no matter what the switch position is
: The back-up lights do not operate off the headlight switch. Either a switch on the column/console for automatic or a switch on the transmission if manual.
4. I unplugged and plugged back in all of the harness connections (at fuse box, at dome light/power, and at tail light harness).
: We have had previous instances where the fault was the dome light. I believe it was grounding out. Look at your wiring diagram and you should see how it ties into the lights.
When I connected the battery, all of the tail lights were on, none of the parking lights.
: Have you checked the brake light switch under the dash which the pedal rests up against and activates the brake lights? The switch can go bad and it is adjustable. It is generally plastic and has plastic threads which can be stripped or damaged, thus moving the switch out of its position to make contact with the brake pedal and not make the brake lights come on or off.
: Don't discount the front marker lights. You could have a bad ground/short in them that is bleeding over as all marker lights will be tied in together.
Your best investment at this point, rather than pull your hair out and never find the problem, is to purchase the Continuity Tester below.
I bought a Southwire brand Model 40040S Continuity Tester. I believe I bought it at Home Dept, or possibly Lowe's. It has a remote tester that can be removed from the main body and allows you to test wires/wiring from each end of the wire. It is not to be used to test for 12Volt power, just continuity. This would be perfect to test your rear harness at the plug to the tail lights. You could test from the harness plug forward, but will need to disconnect the battery as any 12Volt power source can damage the tester. Here is the PDF and you can read what it does: http://www.southwiretools.com/tools/...o?file_id=1821