Jay on "[Plugin: Car Demon] A couple of issues: theme displacement & carforsale not functioning"

You might try downloading the Car Demon Styles PlugIn and trying the responsive style.
http://wordpress.org/plugins/car-demon-styles/

You could also try downloading something like WoodoJo and using it to add some custom CSS like

#demon-content {
	width: 725px !important;
	margin-left: 170px;
}
.widget-container {
	width: 300px;
}

It isn't responsive, but I think it'll get you started on making it work with your theme.