It is the difference between a light bulb and an LED. The LED is a diode and will restrict current flow compared to the light bulb.(they both restrict current but in a different way) The light bulb will have a low restriction of current compared to the LED. Check your light bulb for resistance in ohms and then check the LED. The LED will be different and that is what the TCM sees.
Right but from what ive read, neither LED or Filament bulb behave as simply as that. Filament bulbs have zero resistance (I think) until current is applied of a certain level. The more current supplied, the more resistance the bulb has and the brighter it gets. The resistance value of a light bulb should be the value when being supplied with it's rated current (I.E. when fully illuminated). If a source supplies less current than is required to illuminate the bulb, then it should have zero resistance.
LEDs flow zero current until they are supplied with a minimum amount. Once they have the minimum amount, they "open" and allow current through the circuit. They have no resistance except for that of the wire connections but they do have a voltage drop once they open so that makes me wonder if just adding a resistor would actually solve the issue, it seems to be that they wouldnt.
What I wonder is if the LED is being supplied 12V or 12.2V or something, LEDs should cause around a 1.5V drop once on which would cause the circuit to drop to around 10.5-11 ish but since they only require a few volts to operate they would still function.
There may be a two fold issue here. If the TCM needs to send a low amperage trickle current thru the circuit for a sort of signal, and I know it isnt enough to illuminate all of the LEDs because only 2 bulbs actually light up all the time, then maybe the circuit isnt completing because the current supplied is too low.
Second issue may be that the trickle signal may be sufficient to turn on some of the LEDs but the voltage drop across the diodes may cause it to be too low for the TCM to interpret, assuming the TCM is looking for a 12V+ signal, it likely wouldnt get that after the LEDs.
I dont know if my understanding of the resistance of an LED is 100% accurate but that seems to be where my research is leading.
Either way, im looking at a couple cheap filament lights to wire in for side markers. Put them in parallel should give the TCM a clean circuit.