I have the OE Blaupunkt 5 1/4" grilles and speakers for the rear. Get the adapter rings from Jim C and upgrade your fronts while you're at it!
http://www.lotustalk.com/forums/f95/fs-adaptor-rings-5-1-4-front-speaker-install-51439/
$30 shipped for the speakers.
http://www.lotustalk.com/forums/f95/fs-adaptor-rings-5-1-4-front-speaker-install-51439/
$30 shipped for the speakers.