I have a North American 2014 Golf TDI w/Nav. It has the original headlights/DRLs which are halogen, and original front fog lights. I did not put in a euro switch.

I'm using a HEX-NET / VCDS-Mobile / VCDS-Cloud since I have no easy access to any Windows machines. (Macs, Linux, and iOS only.)

I got the euro Golf R LED tail lights which ECS sells as "with fog lights but without fog function". (Mfg Part#5K0998004KT, ECS Part# ES#2221162). This set includes wire harnesses to adapt the different plugs. Installation was straight forward.

However, finding the right byte 18 code is a huge problem. I get funky behavior no matter what it seems. The outer set of lights seem to have no issues, but the inner set never behave right. What's more, the left inner and right inner always behave differently.

Right now I'm on byte 21, what I see is that with the headlights on, the red inner only lights up one side. In reverse with the lights on, both red inners light up, but so does a reverse light on one side only. The reverse light on the other side lights up, but only briefly after just switching on the lights, and then it fades out. So I've seen all the lights lit at some point, just never in agreement from left to right, and not at the right times. I also occasionally see the inner reds (on the rear right which is normally out while in drive) flicker one and off for a moment just after switching on the lights. But then in reverse it stays on steady.

21 is working the best so far, but it's very asymmetrical. And also, I don't have working DRLs anymore.

The errors I get in the dash are only about checking my right rear reverse / brake lights. No errors about the left side. I double checked the wires are fastened securely. I don't even see any flicker if I wiggle them around.

I tried setting byte 18 to 0x1F and some other codes but always got even worse behavior.

I mostly have avoided trying any of the codes that people know are for xenon headlights or LED DRLs.

What's the logic behind the codes anyway... I was trying to figure out if the byte is just a bit field and some bits are related to your type of headlight and others for the tail light.

Does anyone know what byte is right for my set up?

I think my configuration must be a bit rarer since it's a recent model year, I opted for what I think are non fog tail lights (though the ECS description of "with fog lights but without fog function makes it unclear what exactly that even means), and because the TDI is just flat out less popular than the GTI.