I don't know. One problem is I have no documentation on these girls. Where I'd start is poke around in the fuse panel with various loads turned on and off and see what voltages you get. May be a bad connection. "In theory" the brake lights should not affect signals. You might try wiggling the hazard switch which is involved in those circuits.
You might "wiggle test" the bulkhead connector both signals and wiper, and pull the sections apart and inspect for damage, IE corrosion, heat damage, etc.
