Have you added led TS/Run???
Have you replaced a TS/run lamp in the front??
Bulb incorrectly installed??? 180 degrees off?
1800C uses a small single 1.4watt bulb that goes across each TS wire--that bulb ties the R and L together...
For normal bulbs it works ok because the TS bulbs are 21watts..
For leds TS the diodes previously mentioned need to be added..
The flasher is on the center terminal of the TS switch..
When pushed it to R or L it closes a TS contact AND opens a position/run light on the same side...
So Left TS seems to be ill and flashing fast--missing a full lamp...Check for orientation.. 1157 front
A diagram from Mr. Buras showing how the 1800C is configured and why diodes are needed when leds are used..
http://tech.bareasschoppers.com/electrical/vtx-1800c-diode-fix/