Hmm, I don't know if this is in Elements, but in regular PS there's Filter>Noise>Add Noise (monochromatic). Not a whole lot, and probably on its own layer blended into the texture. I don't know if Elements works with brushes either, but if you search for "photoshop grunge brushes" or dirt brushes, or grime brushes (especially on DeviantArt) you can get some brushes that can help speed up the process. Natural wear and tear happens on edges usually. Dirt builds up on lower areas (on vehicles).
Just Googling for tutorials can be a great help.
There are also places like CG Society where you can browse what other people have done and their help threads.
http://forums.cgsociety.org/forumdisplay.php?f=14
http://forums.cgsociety.org/showthread.php?f=14&t=129189