Left signal lights do not work on 2005 sts

The wiring harness connects to the firewall and was probably jostled when you reached under the dash. Wiggle the wire harnesses under the dash while you have the key and turn signals on and a helper watching the front lights. When they come back on, then you found the problem harness.

it's the grounds if they're both flashing fast it's definitely grounds take a test light and see if its got power goes through the test light grounded to the frame and if the light comes on then it's definitely your grounds

The signal wire going to the right front has a splice for the dash indicator, a wire branching off from the turn wire and going to the dash. It sounds like that right signal wire is broken or lost a connection before it splices off to the dash, so the problem sounds like it is under the dash, either coming out of the turn signal switch to the flasher or from the flasher to that splice. You may need a multi-meter to check voltages on that wire at various places under the dash. Ascertain which color wire you need to concentrate on. Check for voltage on it with key on and right blinker on-voltmeter will spike to battery voltage, then fall to zero as the flasher breaks voltage continuously, if checked after the flasher. If checked when leaving the turn switch, it should be a continuous signal on that wire. Think you better find a wiring diagram to find the right wire to check. Good luck.

change the light control module. it is located under the dash near your right knee on the drivers side. this is a common problem with crown vics.

change the bulb if still out then check and test socket/ground wiring/wire harness for damage

It could be a bad multifunction (turn signal) switch. The way that switch works, it is possible that it can turn on the front blinker and not the rear one. It also cancels the brake circuit so the turn signal can work with the brake pedal depressed.

However, a more likely cause would be a break in the YELLOW/BROWN wire at the rear of the truck. It is common for the connector to get corroded at the left rear frame area where the bed harness connects to the vehicle harness. If you have trailer connections spliced into the wiring harness, these connections are also prime candidates for corrosion. I would look there first. Inspect these harnesses thoroughly all the way up and into the tail light assembly.

If the wires under the back of the truck look good, then I would check for power coming out of the turn signal switch. The wire at the turn signal switch is plain YELLOW (with no brown tracer). It is Pin (M) on the turn signal switch connector. The front signal is a LIGHT BLUE wire at the turn signal switch Pin (H). If you probe these with a test lamp, they should both work the same except the yellow wire will be hot with the brake pedal depressed and the switch NOT in the left turn position. If there is no power at the switch connector, replace the switch.

have you ever driven off road through salt water or even on beaches it sounds like your dealing with a wire harness or fuse box fault take the car to the auto electrician

Sounds like the converter is bad. Go to http://www.etrailer.com/faq-wiring.aspx and check the testing section. Hope this helps.

the next step and almost certainly the problem is the turn signal switch, have seen this symptom on other years around yours and switch fixed it.

What may have happened is either the headlights wiring or the parking lights wiring somewhere along the wiring fused to the left turn signals wiring. Depending on if the turn signals go on when you put the parking lights on or the headlights on.

If the signals come on when you turn on the headlights than you only need to locate the light green wire that comes off the left turn signal harness.

Next follow that wire the entire way back to the firewall looking for another wire fused to that wire as you go.

If there are no signs of this you need to continue looking on the other side of the firewall all the way back to the turn signal and hazard flasher unit which is a black box that connects to a black harness located next to a grey harness behind the knee bolster.

But if the signals come on when you turn on the parking lights than you will also need to check the wiring from the rear left signal and follow that light green wire all the way back to the turn signal and hazard flasher unit.

This should fix your problem, but if not than your problem may lay within the the wiring between the turn signal/hazard switches and the turn signal and hazard flasher unit.

Also you may have a burnt circuit in the junction block. This block is where the light green from the rear left signal and front left signal meet. Don't have a description but if you cannot find it let me know and I will go out to the yard and look myself and let you know.