light problem

I have a 89 fox and It seems like I have some light that is not working but I can't find out which one. I 'm sorry about this silly question but I don't know anything about cars but I need to understand this because it really bothers me. I don't want to have to bring my car in the shop everytime I have this blinking problem (which is not the first time)
You know when we signal to turn and it blinks twice as fast indicating that a light is not working? So that's what's happening to me when I turn left but when I get out to check which one is not working I can't find. I tried switching the back turning light, put a new one (it seemed like the light bulb I took out was burned) but nothing changed. I kept giving me that double blinking sign.

I'd really apreciate if anybody could help me
(please, be patient w/ me)

Thank you
DK

Is it fast in only one direction? Does it use two bulbs for each side in the rear?

Hi, thanks for taking your time for this.

yes, just one direction (left) and it uses 2 bulbs for each side in the rear but the other one is not burned.

Last time this happened, I took it to the mechanic and he changed one light bulb and said the wiring was inverted so he switched. A few days later, the blinking thing started again (all at the left side). I was lucky enough to have helped a guy on the road who knew what was wrong. He said he just switched the wiring. Now I wish I had asked for the details but we didn't have time to talk. But anyway, maybe there is a trick with the wiring???

There is something wrong with the load on the left side, either poor bulb contact or a bad ground.