I can't figure out this mission. I drive the truck to Starfish island, and when I press L3 to get the music started, nothing happens, so customers don't come over. I read somehwere that it had to be a certain radio station, so I tried it on all of them, even with the radio off, and nothing. How do I get the music started so I can start selling?

www.gamefaqs.com should give be able to you all the info you need.

You can only do it once with a truck. If you turn it off or get out of the vehicle, you have to get a new Whoopie truck.
Simply press the mission button, wait for the music to start, then drive around. When you see people, come to a complete stop, and you will either see pink arrows over their head, or it will say you need to go to another area because no one is interested.
Keep in mind that every four sales you make gets you a wanted level, so the mission will take a while to do if you don't want police trouble. That means, make four sales, drive around until wanted level is gone, repeat. Once you hit around 40, people will start flooding the truck, so you need to prepare to make a speedy getaway if a cop is nearby, as you may end up getting an unwanted level or two.

As a quick tip, though, I'd recommend heading over to the stadium to sell your goods: there's a high density of people and not a lot of cops. It still took me about 10 minutes or so of just sitting there to finally get the 50 (?) sales to complete the asset.

maybe i'm wrong but the problem might be with just the pressing of L3..try pressing it quicker. Sometimes when i use it such as to turn sirens off or on with the cop cars it seems to respond weird and often takes a few clicks to register.

on doing the mission:
i didn't try it near the stadium, but I would recommend being near a pay n spray. Sometimes even when I had only 1 star I would just spray it because there's nothing like wasting your time getting 48 of 50 deals and then having a freakin cop come out of nowhere and bust you.

Sometimes the people just don't come. Apparently, they don't want what you're selling. It's not the station, it's not where you're parked, it's just totally random. Though I have noticed in the nicer areas, like Starfish Island, there are always fewer people who will take advantage of your deals. Try lower income areas.

If you don't get some yellow dots within a second or two of stopping the vehicle, move it and try again. Sometimes, even moving it ten feet will cause previously uninterested people to swarm the van.

That happens to me too.

As far as the mission goes, I did it on my first try. I just drove around slowly, pulled up to a corner (so I could see traffic from all directions, and pedestrians could get to me from all directions), did 4-5 deals, then went to another corner. It took me about 10 minutes, and my wanted level never went above one star.

P.S. - Stay away from rival gangs!

Definitely. First time I did the mission, I was at 49, had #50 walking over, when some gang member came over and pulled me out..........
Second time I was at 48, swarmed with people, and a cop came out of nowhere and ended it again for me.
Boy was I pissed.....

Just quickly double or triple click L3 to get the music to play. Same thing for sirens on police cars, fire trucks, ambulances, etc.