I'm not sure if this has been discussed here before. For those of you who have OBD1 equipped cars (190, 124, etc), here's a handy gadget:

Mercedes Benz Diagnostic OBD1 Code Reader - Mercedes Diagnostic OBD1 Code Reader

I've been playing with it a while, and it works well. It's not exactly rocket science...anyone with a bit of electronics knowledge could cobble this up...but it's a neat package, and comes with lots of documentation. Even if you know how to do this with a bulb and a couple of clip leads, it's nice to have something more tool-like to work with.