You're going to have to have a poke about with a multimeter.
Start at the switch......compare the signals coming out of it when you indicate left to the signals coming out of it when you try and indicate right.....if you have a feed to the flasher relay from one but not the other, the switch is at fault.
If the switch sends the same signal to the flasher relay when operated in both directions, test the relay inputs and you should still find a signal in for left and a signal in for right....if not, there's a wiring fault between the switch and the relay.
Then check the output from the relay.....again, you should have the same pulsing output for left as right......if not, the relay is at fault, but if you do have a signal out for both sides, there's a wring fault between the relay and the indicators.......
Most electrical faults like this can be tracked down by applying that sort of approach and logic to it
FWIW, I'd guess it's probably the contacts in the switch, as relays don't often fail just on one side.
BTW.....best practice on Multi meter testing....first find a KNOWN live (eg directly from the battery pos side) and a KNOWN good earth....then leave that earth testing point in place right through the tests at various places so you don't run the risk of testing in another place and
maybe losing the earth connection, giving a false result
(I tend to test each place in turn, then just dab the pos side back to my known live to double check I've still got a good connection for the earth)
