On my 72,I started with a nice NOS Pioneer 8-track.After a while I got tired of the manual tuner and having to constantly repair the 8-track tapes(pluse I am more of an 80 rock person and 70 rock,while good can only hold me for so long).
Next I replaced that with a Custom Autosound 240W AM/FM USB MP3 retro look radio.That unit sounded like crap,and looked and felt like cheap junk.After the first year the USB port would stop working when It got warm.
At this point I gave up on the retro thing,Cut my extra radio suround to fit a modern CD stereo.I bought a 100.00 Kenwood stereo with front USB port.The look doesnt fit what I was hoping for but it works and sounds great.
If you want to try the 8-track option I still have the Pioneer unit with about 3 seasons of use on it.
On my 73 I started out the same way - with the original 8 track / am. I had purchased an 8-track to cassette adapter off ebay - this combo got me by through college.
That said, I didn't sound that good. I just replaced the head unit with one from Retrosound USA and couldn't be happier. It has USB (for mp3/wma playback) and aux in as well as an AM/FM tuner. It fit well in my dash - didn't have to modify or cut anything. It sounds great. This is the one I ordered:
On my 67 I kept my stock AM Radio. Looks cool, works, and it's original. What I did was I had a Zune MP3 player that I always loved. Naturally, it plays MP3's, but has FM Radio and if you turn it sideways, plays movies & videos. I hooked this up to an amp in the trunk along with 6X9's installed by a previous owner and a 12" sub. Sounds GREAT! I mounted it on one of those window suction things but I'm not too crazy about that, but you can put it anywhere. I'm more than happy with it and it's easy to remove with no mods to your car (other than the 6X9 cutouts). Here's a link to some pics and a thread I did on it. http://www.mercurycougar.net/forums...-to-hook-up-a-stereo-amplifier-in-a-67-Cougar