One idea is to get a refurbished original AM radio such as this one on eBay:
http://www.ebay.com/itm/1946-1947-1...Parts_Accessories&hash=item1c4d5c0f08&vxp=mtr
That will get you the AM radio. Then you can hook up a portable CD player or MP3 player that has an FM radio in it like I have and play it through a headphone amp to the speaker such as this setup I've done.
The earphone signal from the phone doesn't really put out enough to play the speaker at desired volumes so I introduced the most powerful headphone amp I could find.
It is very small but boosts the signal to the speaker to normal listening levels. It runs on a 9 volt battery but that battery doesn't last long running the speaker so I made an external 9 volt battery pack with 6 D cell batteries that lasts a year or more.
This is a picture of my original setup using a CD player playing only one side of the stereo signal through the mono speaker. The mp3 player or iPhone are now in the place of the CD player and the output from the amp goes to the speaker switch as explained below.
Then, the other problem is combining stereo outputs into the mono speaker. For most output, particularly the radio shows that were only mono, playing just one side of the stereo output through the speaker was satisfactory. But some more modern music, particularly the Beatles, had strongly separated stereo tracks so that it sounded weird to just listen to one. Every method I tried to combine the stereo tracks caused distortion. So, I installed a stereo speaker in the dash so that I could hear both tracks together.
Now that I had stereo speakers in the dash, I had to figure out how to switch it between the stereo amp where the mp3 player and iPhone played through and the mono stock AM radio in the dash. If I could have found a quad post double throw switch I could have done it with one switch but I couldn't. So, I used two double post double throw Radio Shack switches to do the job. The two leads from each speaker are connected to the center common terminals on the switch. Each pair of wires from the amp are connected to the terminals on one side of each switch. On the other side of the switch the mono signal from the radio is connected with one wire going to one terminal of each switch and a jumper wire connecting the other two terminals so that when the speakers are played from the radio mono signal they play in series rather than parallel to keep the ohms at the higher rate that the radio needs. The switches are mounted side by side in the top of the glove box and are both switched together to either side for the desired effect.
The lead from the amp to plug into the mp3 player or iPhone is kept in the glove box and just brought out when needed to be plugged in.
You can see the two switches on the left of the top of the glove box for switching the speakers between the radio and the stereo. The right switch is for the glove box light. The blue in the back is the 9 volt power pack.