D.K.
www.bettycrocker.com
Type in fire engine cake. Sorry, I had difficulty linking the site. It includes a video on how to make it. You could always watch the video for ideas and use your own favorite cake recipe.
www.familyfun.go.com
Look under recipes for fire engine cake. It also has a how to video and some good advice from other parents who made the cake.
www.coolest-birthday-cakes.com
For a lot of different variations on the firetruck theme.
One of my favorite cookbooks has a firetruck cake. "The Best Of McCall's A Treasury of Favorite Recipes" copyright 1987 You might be able to find a copy of this book at a library. The cake is pictured on page 179 with the recipe on page 200.
Just a word of advice on food coloring. If you plan to make it red, use paste/gel coloring. You will get a better color with less food coloring added. Wilton makes a "no-taste red." I would also probably use white frosting on the inside parts that stick the cake together and only use the red color on the outside visible parts of the cake to avoid as much food coloring as possible.
If the shaped cakes seem too challenging, you could always find an easy picture of a fire engine and trace it. Then you can simplify the drawing, and pipe a fire engine on a flat sheet cake. Practice drawing the simplified version several times before you do the actual cake. Chocolate frosting works best with black food coloring.
Good luck making a great cake!