Evergreen (and probably plastruct) makes a sheet that is meant to be a small scale metal siding. It is grooved, with the grooves about 1mm apart. Evergreen's item number is 4525.
A google image search will get you quite a few pictures of intercoolers, so you can model the style you like best.
I found an excellent tutorial here: http://www.briansmodelcars.com/tutorials/tutorial/2 This is exactly how I built mine, and it turned out great.