Back feeding through pinched, melted, or crushed wires can cause that.
Might consider pulling fuses one at a time till they go out. If one fuse kills both you'll know electrical incest is at work, if pulling one fuse kills one light but the other remains, then you likely have two separate issues.
Pay attention to the fuse you pull, then follow those wires from fuse box on the wiring diagram (you DO have a diagram, right? ) to get a head start on where you're hunting.
Always pull, clean, grease, and preferably bypass the bulkhead wiring.
My car did similar stuff. Found melted brake light wires caused by **** bulkhead connector, they weren't using protection and spreading electron vd everywhere. Pretty gross.
