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What fuse number is blowing?
There are two
#10 7.5amp << turns on power to Flasher when key is on
#21 15amp << powers the lights
So if key is on fuse 21 is connected to lights, if there was a short in the wiring, turn signals off, then fuse would still blow
I assume you have turned on signals and walked around checking that all lights are working?
And both left and right signals flash fast.
What about E-flashers?
All work blows fuse
Most exterior light issues come from Trailer Light wiring, splices that get corroded and short out, so I would start there.
If you can find the Main tail light connector under the bed at the rear, unplug it
Then try turn signals to see if they still flash fast
If not then problem will be in tail light wiring, or trailer lights
If it still flashes fast then problem is most likely in cab
The multi-function switch(turn signal stalk) can short, have to do some disassembly to check it.
Anything new added to vehicle recently?
Maybe pinched a wire or put a screw thru a wire?