Toyota Repair: Turn Signal Question, trailer wiring harness, left turn signal

Question
I have a 99 4Runner.  Recently I noticed that when I turn on my left turn signal, the indicator arrow blinks and so does the "D" drive symbol on the gear shift as well as the radio lights.  

When I turn on my headlights the left turn signal stops blinking and stays on solid "on".  

I have a trailer wiring harness attached and this morning I disconnected the connector between my left turn signal and the left turn signal wire for the trailer harness.  The problem when away.

Do you think the problem is with the vehicle or with the wiring harness?

Answer
The problem seems to be in the towing harness, especially if it's an add on aftermrket harness. If it's a factory installed harness there have been problems with the converter box, it's located behind one of the tail lights or underneath, it's a small black box that converts the rear lights so that they can work the trailer lights which normally use a single bulb for brake and tail lights, most japanese cars and trucks use seperate bulbs for them. This is why the converter is needed. The entire harness needs to be replaced there are no parts available seperately.