Well, you could have picked a harder country for the players to sneak into... *snicker*
Anyway, between LA and San Diego was never really covered by any sourcebooks, so the best I can really say is to make most of it up based on the Aztlan sourcebook, if you've got it. However, here's a hand:
From Target: Smuggler Havens, there is a listing for the ratings of borders and how to cross them:
Aztlan: Sensor Rating: 10 Flux Rating: 10 ECM: 6 ECD: 5 Professional Rating/Karma Pool 4/4.
Scary, huh? Yeah, like I said, you picked a doozy. Better hope your runners are getting paid well enough. Anyway, the Aztlan border guards usually consist of paranormal animals and spirits. The ground patrol is usually 20 well-armed, highly armored troops equipped with Ferrari Appaloosas, Liebre RPV Drones, Tiburon Patrol boats, Aguilar helicopters, Halcon attack aircraft, and Lobo LAVs.
Your players best bet is to get smuggled in by someone who knows what the hell they're doing. However, if the players try to gun it, they'll probably die, painfully. There best bet is to cue in with some kind of distraction or to sneak in through the outskirts of LA perhaps during a riot of some sort. As for the trip in between, that's up to you, keep in mind that most if not all the Aztlan citizens will be Mexicans as most of the gringos left when Aztlan declared itself an NAN (and then subsequently were removed of that status)
Keep in mind that Aztlan has some weird laws, too. For example, you can have nice body armor, and an automatic weapon carried with you without a permit. The trouble is, the police have much much better weapons than you can muster. Additionally, if the policia injure or kill any civilians or cause collateral damage while chasing you, YOU foot the crime and the bill for it. Fragged up, huh? Despite this, Aztlan is the only nation to recognize free spirits as citizens. The environment there isn't exactly... clean there either. All the products in Aztlan are made by Aztechnology, additionally, nuyen is an illegally traded commodity as money must be exchanged for pesos immediately, and those pesos cannot be traded back for any other currency as Aztlan has a closed economy.