The fourth wire in the plug is a switched power lead switched by the accessory switch on the inner fairing.
Do I have to know which one this fourth wire is? can I just use all of them? I want to make this as easy as possible. I'm going to be replacing the light with the new one, I dont want it staying on and running the battery down. I'm assuming this (the new LED interior light) is a direct plugin? I'll shorten the harness on the tour pack to make it past the fender so I have enough to tuck in, then make the length from the battery to the pack.
Sure, you could just use all four wires straight through. It'll work fine. The only downside is you'll have two harnesses going in to the tour pak then instead of one. One harness for the spoiler and one for the tour pak light. Won't be quite as neat in the back. And the interior tour pak's light's connection will still be under the seat. But it'll work fine.
If you want to combine them it's not a big deal though. With a test light or volt meter it'll be easy to find which two of the four leads carry brake light and running light power. Then using the same light or meter see which of the other two is hot when you turn on the accessory switch. The one that never gets power will be the ground wire.
When you know which one comes from the accessory switch just wire the other three straight through from one end to the other. Then when the harness crosses over the fender behind the battery pick up the tour pak's power lead (its plug is there behind the battery) and run that in to the fourth lead in the harness you're making. When done the harness at the rear of the bike will then be carrying a ground wire, brake lights, running lights and unswitched power. Everything you need for the spoiler and your tour pak interior light (and the lights to shine down on the saddlebags should ever decide to add them).
Inside the tour pak you'll have to share the ground between the tour pak interior light and the spoiler. The others will all be independent runs though. Helping out will be that any connections spliced insde the tour pak will be hidden underneath the fitted liner for the tour pak. That's the fitted liner you'll get if you haven't bought one already. The tour pak "lid organizer" is also handy to hide and support the spoiler harness as it travels across the lid (and you thought you were done buying stuff for the tour pak). For that matter, the lid organizer is just very handy in general use.
It sounds worse than it is (sorry). But it's not a difficult chore to combine the two. Certainly don't have to; not at all. It's just easier and neater to have the tour pak light and the spoiler light all in one relatively obscure detachable harness and connection than it is to have two harnesses draped from the tour pak with the light's connection still under the seat.