There is the possibility that you have lost ground at the splice pack where the grounds for the left rear, right rear and license plate lights meet in the splice pack or where the splice pack attaches to the car. These routinely corrode and come off or lose connection. This is SP 401. SP stands for splice pack. I do not know where in the vehicle it is but you can probably trace it from the ground wires in the rear left and right bulb sockets.
Check that first.
If it's not that, then this gets messy to troubleshoot
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It looks like power goes through the underhood fuse labeled PARK, then on to the instrument panel fuse box - left, then through the PARKLAMP relay also in the instrument panel fuse box.
This relay controls the behavior of both the front and rear lights. When the BCM commands the park lamp relay control to close, it also pulls the relay for the front lights closed. So there are two outputs of the relay, one going to the front lights, and the other supplying current to the rear lights as commanded by the BCM when the park lamp relay is closed.
First of all, are you missing only the rear lights or also the front lights not counting the headlamps?
Assuming you are missing only the rear lights, we need to look at the second branch.
Pin B10 on connector 2 of the BCM seems like it should have 12 volts DC on it at the BCM When the park lamp switch is closed. It is the output from this pin that powers the rear lights.
B10 connects to pin E2 on the instrument panel fuse block, runs straight through and exits through pin A2 of connector 2 of the instrument panel fuse block.
So with the park lamp relay closed by the park lamp switch being closed, you should have 12 volts at pin B10, E2, A2. I suspect you will find that one of those points does not have 12 volts DC on it. Usually BCM problems manifest as much more complex nastiness that affects many BCM controlled functions. More than likely there is a corrosion problem or a burnt wire at the pins, their sockets, or at the fuse block.
Pay special attention to the incoming signal to the fuse box and the outgoing signal. There may be a burned connection underneath fuse box causing the issue.
I will try to tackle the dash light issue tomorrow night. Can you be more specific as to what is out on the dash? Is it the backlighting? Do any of the indicator bulbs that come on at startup still work? Is it just the lights behind the gauges? I need to know specifically what works and what doesn't in order to sort out the schematics and figure out where the problem probably lies. I've attached what I think is the correct diagram for the rear lights. I got the schematic from the charm.li site.
