I am driving down to southern baja, pulling a 26 foot trailer with two ATVs. Does anyone know if I have to declare anything at the border? Or do I just see if I get the green or red light?

Nothing there you need to declare, just push the button. I assume you already have your tourist cards? They do check at GN.

These days you don't push a button. I have passed though a number of times the pass few months and have not seen a red light, or anyone. Seems their new system is to wave at you to pull over AFTER you have passed through the gate on a green light.

There ARE green/red ights at every point where you enter into Mexico. You'll see it at the point where you are slowed down the most and it will usually flash green about the time you are through it. There are times though that you may be directed to pull over for an inspection even though you received a green light. If you look closely you'll see an agent (Mexico agent) sitting up high on a chair and looking into the vehicles as they pass through. I elieve he is the one who signals who gets the inspection wave over.