The problem sounds like a dead short. I would start by inspecting the wiring harness for the tail lights and trunk. There are a few push together connectors which can come apart and allow a hot wire touch a ground. I went through a similar drill with my '65. There are a lot of places where I could not see the wiring and had to feel along the wires with my fingers to find the problem. You will probably have to pull up the back seat to trace the rear wiring harness back to where it connects to the flat ribbon that runs under the carpet.
Good luck. This kind of work is not rocket science, but can really try your patience.
Are you working on a 69 ElCamono? (kamino69er) To help isolate the short you can remove the left rear access panel inside you will find a connector pic1 Separate the connector, install a new fuse then turn on the lights. If the fuse doesn’t blow the problem is with the rear lamp harness. Inspect all wiring. you may need to remove the rear lamp assemblies to access the wiring. You will be looking for a bare or grounded brown wire.
If the fuse blows the problem is either in the dash harness, the intermediate (int) harness or the forward lamp harness. Under the dash above the fuse block locate the int harness connector. pic 2 repeat the new fuse test. If the fuse doesn’t blow the problem is in the int harness. This will be hard to inspect you may end up removing the seat. On the ElCamino this harness is routed under the carpet to the left side panel. Behind the panel it passes through the metal wall into the area between the bed side and the outer sheetmetal. Inspect the wiring you can access. If the int harness is the problem we can go into greater detail.
If the fuse blows you can disconnect the forward lamp harness at the bulkhead connector. Do the new fuse thing. If it doesn’t blow inspect the park light/side marker light wiring, again it will be brown wiring that is likely the problem. If it does blow the problem is in the dash wiring.
It is far more likely that the problem is either in the forward lamp wiring or the rear lamp wiring. You can inspect each harness first. Also If you have recently worked in something associated with the light wiring look there first.
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