Now in its 18th year of syndication, "Car Talk," the weekly National Public Radio show starring Cambridge homeboys Tom and Ray Magliozzi, a.k.a. Click and Clack, the Tappet Brothers, has never been more popular -- or better positioned to win new fans, with an animated TV show in the works and more merchandise for sale than you can shake a stick shift at. What makes "Car Talk" run? Let's peer under the hood of a show that deftly cross-wires auto tips with improv comedy: Contrary to what many listeners assume, the show is not broadcast live. It's taped at WBUR...
