left turn signal is blinking really fast

If a bulb is not working on a side, it won't light up, causing the fast flashing you are talking about. This tells you something is wrong on that side. Sound like the mechanic has you covered.

so, my question is, where is the grounding wire......

Its probably in the wiring harness, black wire. Where it gets grounded out I wouldn't know. It may be just a bad socket for the bulb. There are plugins back there to check for corrosion also.

Find the ground wire using a test light find a 12v + say w lights on and test for good ground if ground checks out socket is crap, if ground is absent jump it by splicing into it and ground it to the body

i have a question i have a 2005 sentra v spec and want to know why my signal lights do not work but my emergency lights do . can somebody tell me please and i already checked all my fuses.

Sounds like the turn signal stick of yours isn't working