We took delivery of our inventory Model X in September which did not come with either the accessory or towing package. I just had the towing package retrofitted. Initially we heard a lot of conflicting information about if it was possible and which package could be installed. Apparently per my Tech, Tesla now has a published procedure for a retrofit. I was surprised when the Tech pulled the bumper off and there already was connection there to the trailer plug to connect to which activates the brake lights. Per the Tech, on the retrofit the parts are the same, the only difference (besides cost) between the two options is if they turn the software on for the towing package. Attached is a list of the parts.