The banjercito windows. There's a sign (the red one) telling you what number they are serving - but no place to get a number. Luckily I walked into the top of the stairs and was next in line.
Remember - copies of your Passport, Drivers License, Vehicle Registration and Tourist Visa. You can throw in a copy of your Title but it's not necessary. Have your Credit Card ready.
Just past this building there is a guard gate that will take a quick look at you as you drive past. If you have the window sticker they just wave you through. Not sure how they handle 'Sonora Only' permits.
It's not for another 6-8-10(?) miles until you run into Mexican Aduana (Customs). This is where they look into your car if you get the Red Light as you cross over the metal strip. The sign says STOP and all the agents were getting warn inside the office -- so I just proceeded slowly -- got the Red Light -- and pulled into the first inspection lane. The agent gave me big smile, said good morning, waited until I opened the rear door (my van), asked what's in that box -- and said goodbye.
Very easy and smooth.