A resistor wired from the signal wire to ground will make the flasher run even faster. Like Kevin said, the resistor needs to be in series with the LED to reduce current and therefore decrease the flash rate.
"In series" means:
a. signal wire to one side of the resistor
b. other side of the resistor to the LED lamp
c. LED lamp case grounded
10 ohms may not be enough to reduce the flash rate much. Depends on the LED and the flasher. A better answer may be to use a solid state flasher that flashes at the same rate no matter the load.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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