IIRC, way back when (remember WTF LOL?) there was an issue with teh resistance in that setup where you couldnt just plug & play to get the blinker on the fender unless you added some stuff - going off memory here - but I believe someone did get them to play nicely together. Could be that you need LED's in both the marker lite of the fender and in the turn signal so the entire circuit was consistent. Might want to check the write up library. Should still be in there.
Reminds me that I need to do the blinker mod. Cant believe they still come from the factory without a blinker on the fender. So damn dumb.
The resistor is only needed on the turn signals so the flasher program in the PCM gets fooled into seeing a normal light bulb and indicating a lamp out (via the quicker flash rate, known as "hyperflash"). It is not necessary with marker lights. Even if you do the blinker mod, as long as your turn signal lights are regular incandescent bulbs, there is no need to add resistors. You need to put a 6 Ohm 50-Watt resister in parallel with each blinker light circuit so the PCM detects enough current draw to consider the "bulb" to be good.
The bad news is that when you do this you will not get any notification if the LED fails for any reason, such as damage to the LED "bulb" or wiring.
Backstory (because I like impressing people with my encyclopedic knowledge of useless trivia)
In the old days mechanical flashers used the circuit resistance with a capacitor to set the flash rate on turn signals. When a bulb burned out it changed the resistance of the circuit and caused the remaining bulb and dash turn signal light to flash faster than normal, a condition known as hyperflash. So when your turn signal indicator went dot-dot-dot instead of dash-dash-dash you knew something was wrong and either replaced the $1.00 burned-out bulb or went to a garage or dealership where they would charge you $75 to flush-and-fill your blinker fluid.
LED's do not have the same resistance as incandescent bulbs. Actually, bare LED's themselves have essentially zero resistance at 12V, acting like a dead short across the power supply until they pop from the heat (well under a second). Internal resistors are added to vehicle LED "bulbs" to limit the current (Amperage) to safe levels. This total resistance
(LED + built-in resistor) is 10X to 100X higher than an incandescent bulb.
When aftermarket LED upgrades first came out folks discovered the problem and electronic flashers that did not rely on circuit voltage for flash timing were developed to solve the problem. But the thing is that because these electronic flashers operated at a constant rate independent of the circuit resistance, people wouldn't realize that a blinker light wasn't working until someone told them. Of course LED's don't easily burn-out, but the wiring or LED itself could get damaged.
Then later in time the flasher controls went to computerized electronics instead of mechanical flashers. I guess for some reason it was better for consumers to have hundreds of dollars of non-user-serviceable computer power and complex, real-time programming to flash their turn signals than a simple, easily-replaced $10.00 flasher. I digress in my digression.
The engineers weren't dummies and they knew about the problems with the early, simpler electronic flasher modules in notifying the driver of a burnt-out bulb. So at great expense and even more complexity and downstream cost to the consumer, they included current sensing circuitry and monitoring programming to measure the amount of current being drawn by the blinker bulbs. When that current goes lower than expected values, the programming is designed to "think" the bulb is blown out and either produce the "hyperflash" blinking or turn on a "Lamp Out" warning light to alert the driver that something is wrong. (On the JK you get hyperflash.)
So when you replace a 27-watt incandescent bulb (what a typical 3157 turn signal mode draws) with a 3-watt LED, the JK CANBUS doesn't sense enough current draw to believe the bulb is working. This results in the PCM triggering <left|right> <front|rear> TURN SIGNAL SENSOR SIGNAL LOW condition and activating hyperflash to notify the driver. Placing a 6-Ohm resistor across the bulb's turn power and ground lines (i.e., in parallel to the filament) increase the current draw to a high enough level to "simulate" a working incandescent bulb and avoid a problem. The resistor has to be rated for 50W to keep from burning out. It is also a good idea to mount the resistor to a metal surface inside the Jeep to help dissipate heat. Normal turn signal use shouldn't generate enough heat to be a problem, but if you are using your hazard flashers for an extended period the resistors can get quite hot.
Finally, also note that there are "corrected" LED "bulbs" that draw the same 27W as a standard 3157 incandescent. They incorporate the necessary resistors internally to simulate a standard bulb. I'm not sure how long last compared to plain LED "bulbs" with external resistors.