92 Isuzu Rodeo-turn signals

The left turn signal acts as if a bulb is out; replaced bulbs on left side, replaced flasher, checked all readily accessible wiring- still will not
work- 4 way flashers work and you can get left turn signal to work if you turn it on while 4way is flashing, then turn off 4way. Problem started after I added trailer wiring harness but dealer has rechecked that. Thanks. Cb
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