You don't have a choice about pushing the button. As you approach the aduana, you have the option of using the declarations lane or not. If you choose the former, you will tell the agent what you have and then pay the appropriate duty. You will then be directed to push the button and, if red, your car may be searched in order to determine that you actually paid for all the stuff for which there is a duty. That is the choice most gringos make but it is not, in my father-in-law's words. "the Mexican way". Evidence of this can be found in the parking lots at Chula Vista shopping malls and Costco. They are full of shopping carts which contain the boxes and packing materials of new items particularly electronics. Mexicans and savvy border-area resident gringos frequently do this in order to be able to say that they are used merchandise. They know that there is no penalty for driving through the nothing to declare lane and then being given the random red light. If that happens to you, you will be directed over to an area where your vehicle will be searched irrespective of what you say you have got. If you owe duty, you must pay it but, as I said, there is no penalty simply for using the non-declaration lane. This information is based upon five years of daily commuting between Tijuana and San Diego. My suggestion pertains to border crossings in general. With a 28 foot trailer, and the increased awareness of arms smuggling. it is possible that you would get waived over for inspection even if you do get the green light. I am not aware of any recent changes so suggest you just do whatever makes you the most comfortable.
P.S. Gasoline is much more expensive in Mexico right now than it is in the United States. When I used to drive down to SMA from San Diego, I always chose to drive east on the 10 through Arizona, New Mexico and a very long and boring stretch through West Texas. From San Antonio, drop south on I-35 (don't say "the 35" or the natives will know you are from Cali) and cross at Laredo. The US roads are in good shape but unless you have satellite radio, you might become bored after hours of religious programming. From Laredo you follow mostly smooth and fast, but very expensive toll roads almost all the way to San Miguel.
(This post was edited by Brian on Jan 11, 2009, 2:33 PM)