Need suggestions for '66 El Camino rear speakers

Hi,
I'm upgrading the stereo in my El Camino. I'll be adding a 50W stereo/CD. I have 3" speakers in the kickpanels in the front currently, but would like to upgrade these speakers from the cheap ones I currently have installed.

Further, I would like to add rear speakers. Unfortunately, I have no idea what might be the best setup to use, as mounting location and available space are the big issues.
I would be interested in seeing what others may have done that work well. Perhaps some small speakers could be mounted on the metal "package tray", perhaps "wall-mounted" to some sheet metal in the rear of the cab, perhaps below the belt line on the rear kickpanels (if that's what you call them). Of interest, would be any information regarding specific models of speakers and enclosures that would be suitable.

I'm not going for an all-out stereo; I'll reserve that for use in my home. I do want something decent, though, so I'd be willing to spend up to $100 or so for rear speakers.

Thanks for any help.
Tom