The bulbs are simple, quick, and easy to check. Not to mention, inexpensive. Go here first.
Eventually, you'll need to replace a bulb anyway. Don't forget to replace both the front and the back. If that does not solve the problem, THEN consider the multi-function switch. However, check for a faulty ground before you look to the switch.
A multi-function switch is not a simple piece of electro-mechanical workmanship. By that I mean, it can fail and can fail over a period of time in unpredictable fashions.
Last year I had my mechanic replace the multi-function switch in my '99 Ranger. He charged me $100.00 including parts, labor, and a cute little air freshener with his garage name on it. But, that was only after I swapped out all the bulbs in question, and traced back all of the ground wires in the harness. You'd not believe all of the nonsensical, sporatic symptoms I had with the brake lights, turn signals, and hazzards until this switch was replaced.