LED bulb is more like a switch in the on position, my mirror map lights always glowed in Oldblue, the rear dome light worked like normal with LED bulb .

 

Sounds like a back feed to me.

 

Do you know if the dome light was an error free LED as that could account for it not glowing?
My theory is that since the BCM controls ground there is probably always some very minor continuity to ground for the lights which would allow the LEDs to glow since they are so efficient.
My hope is that maybe the diode would possibly interrupt that enough to stop the glow, if not surely the relay would.

 

Actually, the wiring is a bit more complicated than you think. The BCM supplies both sides.

This is for 2006:

 

I could be wrong on this (happens on occasion) but I don't think the BCM has any actual control over the positive side of the circuit, it merely passes thru.
Those "not used" circuits are good to know about tho.

 

One is "supply" and the other is "control" (not ground) it dims the lights. A LED is a switch that glows when it is "on", they all have a threshold, the resistor handles that. Incandescent lamps are on/of "appliances".

Do you leave the map lights on for hours at a time? One little resistor is not going to get all that hot, not sinking much.

 

"Supply" is positive battery voltage.
"Control" is the ground portion of the circuit.
That much is obvious just by looking at the way the diagram is laid out. If "On" and "Door" are on the same side of the switch for the cargo/dome lamp and since "On" works by directly grounding the circuit then "Control" must also be a ground path but instead of a direct On/Off like most switches it is a variable resistor of some sort (or PWM) in order to "dim" the lights out. As such it never really breaks the circuit entirely, it just raises the resistance of the ground path to the point where an incandescent bulb no longer has enough current flow to function.
I may get out my lab scope this weekend to check, really hope it's not PWM.

And yes, sometimes I do leave the lights on for extended amounts of time, either accidentally or purposefully if I happen to be working on something in the car.
For some LEDs with resistors it does not take long to get hot either. The resistors can reach temps of over 200c (almost 400f) in a matter of minutes in open air environments. Put them in an enclosed space such as a map light or cargo/dome lamp fixture and it surely will only make the situation worse. Heat is also a killer for LEDs so if I can run LEDs without a resistor not only will I keep more of the advantages of LEDs, less heat and less current draw than incandescent, but the LED should last longer as well.
I have never checked the temp of the map light bulb but I have checked a 3157 with just the tail light portion working and the bulb was up to 300f in about 5 minutes, never checked with tail and brake light both working to see how hot it got.
I do have an older car that has LED turn signals and I use load resistors on it. I guess I should check the temp on them just to see how hot they do get.

 

Control may actually be ground but not exactly. These are computer circuits where low ref is not exactly the same thing as ground.

[QUOTEit just raises the resistance of the ground path to the point where an incandescent bulb no longer has enough current flow to function.][/QUOTE]

That is my point! But enough to switch a LED on. Thus a 220 ohm resistor, I don't have the math but my bet is that dissipates less than a fraction of a watt. You are over thinking.