Really old post, sorry but did you ever figure this out. I have 2001 silverado. Almost exact same problem. New LED flasher, all turn signal LED work, side markers don't come on unless daytime running lights are on and the turn signal is activated. Then they blink like they are supposed to, but if the headlights are on they don't blink or come on with parking lights . When the headlights come both turn signal indicators on dash are on faintly although turn signal blinkers still function.

 

I just went through the exercise of installing LEDs for park, signal. brake and backups on my '96. Separate brakle and signal (amber signals) I bought several different kinfs of Flea-Bay LEDs - and I founf there are LEDs and there are LEDs!!!!.

The Trux brand from Princess Auto worked fine -other than hyperfloash - but were so dim they were useless. They work by lighting some leds for fim and all for bright. The first set of Flea-Bay 3157s wouldn't even fit the sockets. I tore one apart and it had no current control circuitry other than 2 resistors - a 110 and a 220 ohm - tied together so for bright power goes through the 110 and for dim throuigh the 220. They didn't fit - and I didn't like them even if they did, as they would likely have burned out rather quickly.
The second set fit and were advertized as non polarity. Since they didn't work if installed "backwards" they were NOT non polarized - and when they were installed right for the rear signals they worked perfectly (as 3156 bulbs). Same for backups. In the tail- brake they lit bright as tail lights and did not get bright with the brakes and tail lights on.and had other behaviour issues. Front signals also malfunctioned, and with the signal on my radio display flashed dim to bright.

I pulled a bulb apart and it had a current limiting circuit, but it still had a resistor on one terminal to provide the "dim" output - and nothing to prevent power from the bright terminal from feeding back to the dim, or vice versa..

The SOLUTION is to get 8 3 amp 20 volt or higher DIODES. Installation involves cutting the bright and dim wires and soldering diodes in series with each, with the cathode (bar marked end) towards the bulb. I soldered and heat ecrinked them into the harness and taped them all up. They need to go on all of the "dual filament" sockets - on the '96 that is the tail/stop on the rear and the park-directionals on the front.

I THINK the backfeeds were responsible for damaging 2 EP27 flashers. They now click continually even without the signals or hazards on and will flash the directional signals with the key off. I'm fixing THAT by installing 2 2 terminal electronic flashers wired into the stock flasher socket. They are on order.

The diodes are PITA but will allow any style of 3157 bulb, LED with or without resitors - or incandescents to work.

 

Hey thanks for the reply. To be honest you kind of lost me in the fourth paragraph. I did solve this problem although it was much easier than I thought. The new LED flasher is key. The side marker lights as others have mentioned are single filament. The polarity in the side marker reverses when headlights are on versus off. I solved my problem by purchasing bipolar side marker lights super cheap!!!!!! I changed my turn signals to leds that are solid white running but yellow when blinking. Not sure it was necessary but it worked. The side markers now light when parking and headlights are on and blink back alternately with the front turn signal bulb like they are supposed to, no faint turn signal indicator either.
Here is a link to the lights that fit my chevy. dual turn signal and running light hopefully a similar bulb will fit the Ford community.


side marker light link


here is my review with a video of the side markers working with this relatively inexpensive marker light .


I would bet Ford lighting scheme is almost identical as the symptoms of placing unipolar leds in the turn signal seem to make side lights and turn signal indicators malfunction.
Hope this helps someone out there.
God bless.