Sosa called Tony to drive a car bomb expert around New York to take out the man putting the dealers' collective balls to the fire. Coupla different problems with that:
A car bomb only works if you're trying to send a message, which is why its favoured by guerrillas. If you're trying to take out one guy to shut him up, a sniper rifle's a far better choice. Alternatively, they could have paid someone to shoot the guy, while they were watching him walk from his car to his hotel.
Tony's a highly skilled drug dealer who has a problem harming innocent civilians. Why would you get him to do it, and not just pay some random guy $2000 to do it? And for that matter, why not just find an English speaker who knew his way around an explosive charge and a remote control? The job didn't exactly require a great deal of specialisation.
Tony's explosively violent and as high as a kite 24/7. Why on earth would Sosa give him such a delicate job?
The cartel is trying to send a message. If they just shot him, what was to say there wouldn't be a dozen more guys just like him trying to take them down the very next day? You can't hire "some random guy" to do a hit. It had to be someone within the organization. Otherwise then you would have to end up killing him as well, to make sure he didn't talk. Tony was selected for the job because Sosa wanted to know that Tony could be controlled, and would fall into order.
Assuming Tony can do that, which he doesn't, and everyone gets fucked over. Sosa less than others, though. Still, there's really no reason Tony specifically has to do it, and the deal is overwhelmingly in Tony's favor if he does. If anyone else who speaks English and is in Sosa's organization does it, the job gets done. All Sosa gained from asking Tony was keeping a trading partner out of jail for 3 years, and even then, he has to call in favors to do so.
Sosa seems to really like Tony, though. He probably had no idea how far Tony had descended into drug addled insanity, either, since they live in separate countries and Tony's organization seems to continue to function just fine. Sosa isn't the type to do things for people for free, though, so he naturally needs a favor in return. He probably saw it as a two-birds-one-stone kind of deal: help his friend without doing charity work and kill this asshole activist. No wonder he sent an army to kill Tony; he was furious at being betrayed by someone he genuinely considered a friend.
Why did they not blow up the car the second the victim gets in? Why wait? This smells like a Plot Device to give the family time to get in with him, and let Tony make a decision that will lead to his downfall.
It was specifically stated that the car was to blow up in front of the UN building. This was part of the message implied from the above topic, so it would discourage witnesses ranging from future reporters to politicians that do have the power to put a stop to Sosa and other dealers the reporter was whistle blowing.
How the hell did Sosa get that many gunmen to Tony's mansion at the end without anyone noticing? There must have been almost a hundred of them, and they presumably came in less than two days after the botched bombing. How did Sosa get that many men from Bolivia and Miami with their guns and with no one noticing something? How did they even expect to get away before the police came a knockin' with the kind of noise they were making?
A private plane, perhaps? Convoy? Ship? There are plenty of possibilities. As for the last question, Tony lives in a heavily guarded private compound in a fairly remote area. It's entirely possible that if there are any cops around at all they've either been bought off or have learned to adopt a "Just leave things alone unless they spill out of the compound into the larger area" mindset.
Tony and most of his crew were away looking for Gina. They were already over the fence when Tony brought her home. So taking out the guards was a piece of cake.
One of the things that bothers me about the movie (and it has been a long time since I've seen the movie, so it might have been explained), but why did Tony even start getting high on his own supply in the first place? I know it may have been boredom, but he has people trying to kill him all the time so they can get his empire, and all sorts of activities that could land him in jail or get him killed at any moment, can't be that boring in that case. Also, secondly, why does Elvira also get addicted to coke? She was the one who told him the second rule: Don't get high on your own supply.
What threat did the car bombing target pose to the cartels, exactly? Apparently the guy was pretty well-known and had gone on TV to talk about how bad drugs are and how much money the dealers make. Well, everybody knows that already. Did the cartel leaders think that if this guy got to talk to the UN that suddenly the whole world would end the drug trade? And if they just wanted to "send a message", then what message was it? That drug cartels are violent criminals? Everybody already knows that too.