Hello. I have a 1997 F350 7.3 turbo diesel dually, 2 wheel drive automatic trans. I'm having malfunctions when using the turn signals. Right now when I use them at high speed the transmission down shifts and the cruise control disengages. Also, at all times, when using the signals, all rear lights flash, including third light (front lights operate normally). The dash lights operate normally except they stop blinking and become constant when braking.
Could it be a bad multifunction switch?. Bad trailer light wiring?
Any help is appreciated?

I'd check the trailer light wiring, combination light switch, and emergency flasher.

Did you recently add something to the wiring? Like a trailer harness adapter? Stereo or amps, etc?

Thanks, the problem was a 7 to 4 pin adapter I left plugged in.