[olddog]

In the old days they used a flasher unit. They plugged into a spot in the fuse box. They were about an inch in diameter and an inch tall. They are basically a relay with a high temperature cut out. The current being pulled by the bulbs heated the cut out up and the relay opened. When it cooled, it closed again. Thus the flashing light. When you connected a trailer, the extra lights caused them to flash to quickly, so they sold a heavy duty unit for people who toed.

I haven't replaced one in decades. I do not know if the same technology is used today or not. I have had 3 Ford products that I kept for 12 to 18 years and put up to 260K miles on them and none have ever failed on me.

If it is temperature related it pretty much has to be the flasher unit, assuming all the lights are working both hot and cold. It also makes sense that in cold weather your bulbs are not pulling enough current to heat the cut out up enough to switch open. You haven't changed your bulbs to LEDs have you? They do not pull nearly as much current as a standard light bulb.

Hope that helps.