left signal quickly

wash battery
wire harness socket connection
the light itself ---right lamp to left
wires have chaffed bare somewhere
removing the lens to see
a mess of vapour control stuff (gas tank associated) in that area. May need to get at it from underneath by pulling the horizontal flap down (held in by a plastic push connector)
a missing ground, a weak ground, or an intermittent ground.
inspect all your light sockets for how well they are grounded to the tub
The bulb sockets receive their grounds through the three tail light enclosure screws that hold it to the tub, make sure they are providing a good ground. In fact, that ground is often screwed up by having mounted those aluminum tail light guards which can cause very hard to diagnose electrical issues.
Mine was doing that and I took the light cover off and tightened the bolts to make a better ground connection and I have had no problems since.
Bingo! It was a grounding issue. I removed the light cover and tightened down the 3 bolts that hold the housing to the body of the Jeep (I bought the tail light protector and they gave me 3 new bolts, and they all had a piece of rubber in between the head of the bolt and the lock washer. I didn't tighten it down enough, so the rubber insulated the ground).
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Follow the wiring harness from the headlight (and parking/turn signal lamp) you will find a "branch" off of the harness going to the body(ground) of the car. There is a "branch" going to ground on BOTH sides (left/right) behind the headlights. If where the wire lug is bolted to the body is corroded (may look good) under the bolt or serrated lug you will have the problems you are describing (dimmer lamp and fast flashing).
When the headlights are turned on they draw more current through the same ground return wire as the turn signal lamps and the corrosion limits the amount of current available to the lamps. This is similar to the use of cheap "jumper cables" in winter....the headlights dim as you crank the engine over due to the small diameter copper wires (or aluminum wires in the real cheap cables).
You can "test" if the ground wire(s) are bad by running a wire from the negative (-) (black) terminal of the battery to the ground wire of the turn signal/parking lamp. If your turn signal lamp gets brighter and the flash speed slows to normal....go looking for that bad ground. Remove the bolt and scrape the area of the body where it was bolted, then smear n (available at Advance & AutoZone) all over the area, wire lug. and bolt. Then re-assemble. You should now have solved your problem.

Spray a little WD-40 on a rag or cloth and run over the wires as you go along to clean debris and oil off the wires and offer some lubrication and protection as well.
Read more: How to Preserve and Protect Electrical Wires Around a Car Engine | eHow.com http://www.ehow.com/how_2283220_preserve-protect-electrical-wires-around.html#ixzz0tcvfSXTB
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Disconnect the negative cable from the battery. Loosen the retaining nut on the clamp that is secured to the negative battery terminal. Then, slide the cable off the terminal.
Step 2
Open the hood and locate the turn signal lights. They will be located on the outside of the headlights towards the fender.
Step 3
Remove a section of protective coating on the wiring for both the power and ground wire running to the turn signal. You'll need to use wire strippers for this. Remove a section of wiring that is close to the subframe where you will mount the resistor.
Step 4
Solder a section of 18-gauge wire to each end of the resistor.
Step 5
Solder the other end of each wire you just soldered to the positive and negative wires for the turn signal. Since this is a resistor, polarity does not matter (it doesn't matter which wires on the resistor get soldered to which turn signal wires).
Step 6
Place a liberal amount of heat sink compound on the bottom of the resistor and secure the resistor to a free area on the subframe with safety wire or high-temp electrical tape.
Step 7
Repeat steps three to six for every turn signal light (including the rear lights). You must do all lights for the fast blink LED problem to be solved. Each light must have increased resistance.
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Lie down in the floorboard of the driver's side of the car and look up under the dash with a flashlight. You are looking for a small silver plug. This is the flasher. To make sure you have the right flasher, turn on the turn signal and feel the casing of the flasher. You should be able to feel it clicking.
Step 3
Remove the silver plug and replace it with the new flasher. Slide out from the floorboard and start the Galant. Recheck the turn signals and make sure that they flash, as they should.
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Start at the battery. Make sure the terminals are clean and that the cables aren't corroded. Follow the negative cable to where it connects to the block and make sure that connection is also clean and tight. You'll also probably have a connection to the fender, and again, clean and tight.
There should also be a ground strap from the rear of the motor to the frame and/or firewall. This is usually on the passenger side with GMs and it very often corrodes and falls apart.
特意找了点资料了解了一下原理：转向灯的闪烁功能是通过频闪振荡电路来实现的，其核心部件是闪光器，正是通过它来控制电流的周期性的通断（闪光频率规定为1.5HZ+_0.5HZ），从而实现闪烁。常见的闪光器有电容式，翼片式，晶体管式3类。这3类闪光器的特性有所不同，故障监控功能也不一样。当一侧转向灯有一只或一只以上转向灯泡烧断或接触不良时，电容式闪光器就使该侧转向灯接通时只亮不闪，以示该转向灯电路异常，以前的国产切诺基就应用了这种电容式闪光器。翼片式和晶体管式闪光器结构简单体积小，闪光频率稳定，适用范围更加广，它的监控作用也更加明显，当转向灯出现故障时，闪光器的振荡频率就会发生改变，闪烁频率会明显加快，正使用这种特殊的方式，提醒驾驶员该侧转向灯电路已经出现故障。现在，大多数的车型采用了晶体管集成电路闪光器，现在还有CAN总线传输技术。如果某个转向灯出现故障，车身控制电脑会让相同位置的其它灯临时代替转向灯的工作，并且发出明确提示，通知驾驶员哪个转向灯已经发生故障，需要及时检修！！！

那是因为有坏的灯泡，或者有短路，或者是继电器坏了

三,可能是你换过转向灯,但换上的转向灯比原来的灯瓦数小些. 解决半法:一,三更换灯泡.二更换闪光器．

4.把转向灯拧下来（方向我给忘了 都试一下吧）
5.用干的摩擦力比较大的东西擦一下灯头（就是后便接触电源那）
6.从新安回原位.

应该不是转向开关的事，你两边都开一下看那边闪的快，如果两边都闪的快那就是说转向继电器坏了，如果一边闪的快就是灯泡坏了，坏那换那就得了！
转向灯接触不好，拆下来重装一下，或是把接触弹片较一下就行了。
如两边都快，请检查转向开关，及闪光继电器。