Brake light wiring

My left rear brake lights do not light up. I checked the wiring harness in the trunk and there are no breaks all the way up to the coupler which is located on the right side fender. The ground appears to be fine, because if I run power from the right turn signal with a jumper wire to the yellow wire which powers the left signal, the bulbs light.

I have a manual on order, but for now, I have no idea where the wiring goes once it leaves the trunk, and I am a bit hesitant to just start tearing things apart.

Anyone have a diagram or an idea of how to follow the yellow wire to check for a break somewhere?

Oh, 89 GP.

Thanks.