It made me curious so I looked around. It sounds like one of those things that varies depending on factory specs.
Resistance could be added to reduce radio interference on some wires.
Here's some of what I found:
http://www.tpub.com/basae/44.htm
http://www.inct.net/~autotips/plugwire.htm
http://www.babcox.com/editorial/ic/ic120214.htm
-Mike
I'm right at home in a filthy garage