Ok to get the boat car, you have to play the squidport level with
  Lisa. During one mission u find Bart on a boat. After u talk to him
  jump up onto the ledge and explore the deck of the boat. Along with
  finding a collector card, u should also find a boat car across from
  the collector card.

  Now to find the Monorail car, u have to play the downtown level with
  Marge. During one of the missions, you'll come to a broken monorail
  track on a small street. go up the stairs to the monorail and jump
  onto it and jump in the first car. Then u can drive the Monorail car!!
  There will also be a collector card there too.

  


  
  


  

At the options screen, hold L + R and press X(4). If you entered the code correctly, you
will hear the sound of a horn.

First go to the 3rd level and go to the huge boat in the
  harbor(c-spanker).Then go around and you should see a house, (Davey
  Jones Hamper). On the left are 2 crates on top of each other. Jump on
  the chain rail and then jump behind the crates. Walk in to the wall
  and your in the glitch house.

  

When you are about to start a new game, go to options menu and enable the "Fast cars" and
"Red brick car" codes. In Homer's first level, get the rocket car and it will be the
little red brick car. Get in it then hold L until you are at a complete stop. Keep L held
and you will start to skid flames for a moment.

At the options screen, hold L + R and press A, B, A, B. If you entered the code correctly,
you will hear the sound of a horn. Note: You must have first completed all
missions.

At the options screen, hold L + R and press Y(2), B, X. If you entered the code correctly,
you will hear the sound of a horn.

- Red Brick Car

Go to the Halloween level aka level 7.  Now walk out in to the
  middle of the street.  And wait till a car stops in front of you. 
  And when it says press (y) to enter. Don't do it.  Instead, press
  (a) and jump on the back of the car that stopped by you.  It
  works best with the ghost boat because, there is a lot of room on it. 
  Once your on the car, don't press anything.  And after a few
  seconds, the car you are on will begin to move and homer will act like
  hes surfing.  (The car will take you all over the level)

  

Collect all cards in the game then go to Lisa's level. Go to the Android's Dungeon. Talk
to the Comic Book Guy and buy a ticket for the cartoon. Then, go to the Aztec Theater to
watch the cartoon.