MG Car Repair: Turn signal, turn signal switch, jumper wire

Question
QUESTION: My turn signals work fine when the parking lights and head lights are turn off. When the parking lights are on 3 for the 4 turn signals work. The front left does not flash, tried changing out the bulb, but that did not help. Any ideas?
Thanks, Tom

ANSWER: Tom, you will have to tell me what kind of car this is.
Howard

---------- FOLLOW-UP ----------

QUESTION: It is a 1979 MGB.

Answer
Hi Tom,
First you need to test the grounds at all parking lights front and back. If they are good go to the turn signal switch plug and unplug the switch and take your wiring diagram and test each turn signal light with a powered jumper wire. Starting with the left front.
Howard