Toyota Repair: electrical 2000 toyota tundra, control lamps, tundra 2000

Question
2000 Toyota Tundra left signal stays on when park lamps or headlights are on.  When park lamps and headlights are not on or taillight fuse is removed or headlight switch is unplugged, left signal works normally, however, dash lights, radio/heater control lamps also flash. All ground wires seem to be ok.  Command start disconnected, no change.  Rear tail light wiring harness connectors unplugged and inspected, seem to be ok.  All sockets and bulbs seem ok.  Turn signal bulbs were swapped from side to side, problem remained, left signal stayed on, right worked fine.  Flasher relay was checked and seeems ok.  All fuses were pulled and checked.  Turn signal and headlight switch, removed, contacts cleaned and inspected, look ok.  Any suggestions would be appreciated.

Answer
Does the truck have a factory OEM trailer hitch and towing harness? If so, there was a special service campaing(SSC10H) that involves replacing the trailer towing harness at no cost that corrects these electrical problems.
If there is no trailer hitch installed let me know and I'll try to come up with something.