You already plugged the "Driver's Side Assembly," into the "Passenger Side," and it operates correctly. Question: Did you try the "Passenger Side Assembly," on the driver's side ? Try doing this and if it operates, than it's not the "Multi-Funtion Switch MFS."
The "MFS," also controls "Hazard Flashers," and "Headlights,", are they working properly ?
You also claim the "Signal Bulb" illuminates but does not blink. So, the first thing I would check is the "Bulb Socket,' make sure it's clean, and replace the "Turn Signal, Bulb Type 3157." Could very well be just a "Bad Bulb."
I agree with you, eliminate the small things, because a new "MFS," is well over "$400.00." Unless you pick up a used one at a wrecking yard for around $75.00. Installing the "MFS," is an easy task, that should take you about 25 minutes.
I really think it's a "Bad Bulb," or a "Bad Ground Connection In The Socket," but heck I also didn't think Obama was going to be elected President.
Good Luck !