Here's my thoughts on the issue:
1972 never came with door speakers, so there's not really any way of wiring them, at least not a clean way. Though if anyone has any suggestions I'm open, because I'm installing new carpet and RS door panels this winter. Right now I have 2 mid-grade speakers on the rear parcel shelf and 2 mid-grade speakers in the dash (where there was originally a single speaker). Sound quality is poor, to say the least. I'd like to improve sound quality without a) spending $1000 and b) installing all sorts of crap in the back.
My hope is that this under-dash unit will help with two decent size speakers up front, plus get a small sub to put under the passenger seat, something like this:
Kenwood - KSC-SW10