If you are going to use your original AM radio, you can't use modern aftermarket speakers. Modern speakers are 4 ohm, while vintage speakers are 10 ohm. You can get reproduction speakers, 8- 10 ohm, here: http://www.turnswitch.com/speakers.htm You'll have to run a parrallel line off your dash speaker to the back. The service manual will show where the optional rear speaker was mounted, http://www.1968firebird.org/illustrated/misc/antrad1.html however, that speaker was only intended for FM radios.... don't expect your AM radio to drive 2 speakers easily- its very low power. If you want better quality sound out of your AM, I'd concentrate on reconing or replacing your dash speaker. If you want un upgrade, but still want it vintage, I'd look into an AM/FM stereo from your same year. If you want modern good sound without cutting too much, check out Custom Audiosound. If you want great sound and don't care about originality, there are infinite options, but you'll have to modify the interior.
