Rear lefthand side brake light not working, put a brand new bulb in & still not working even though its working wen i have my lights on???

Please try another bulb before you go for something else. Many times happened to me even the new bulb not working. Advice: just remove the good bulb of brake light you have on driver's side and try it on the passenger side. If it works,surely another bulb. If not,ask for an Electrician to help.

Your rear left side combo lost a ground on the part of brake/signal lamps. Just put one more wire between parts.

make and model of car?

With the age of the car it could be that your grounds are bad somewhere which explains why essentially no lights work ... even though the bulbs/fuses are OK. You get power to these lights by having positive wires going to the light bulbs and the negative side of the circuit coming from the frame of the vehicle (referred to as the ground).
Pull all the connectors that you can see and make sure they are not corroded.
Check also that no associated relay that has anything to do with the rear lighting is not bad.

The brake light side of the bulbs is powered by the brake switch and travels thru the turn signal switch.
If the third brake light works, I would guess the problem is in the turn signal switch.

did you check you fuses or relay switch?

You might need a new Brake Light Switch, it is located by the brake pedal.

If the blinker and reverse lights work on the bad side maybe a new socket.

Check the 10 amp "Tail" fuse. This controls the running lamps.
Note that the rear bulbs are "dual filament" bulbs, and even though the brake lamp feature works, the running light feature in the same bulb can be bad.