Well, I decided to put in a modern Pioneer CD player. I trimmed the H frame with an air die grinder and cutoff wheel and installed a couple of 5-1/4" speakers in some homemade kidney boards attached to the side of the H frame etc. I have Miata seats with the high end speakers in the headrest. I installed a 100+100 watt amp I had laying around to power the 5-1/4" speakers. The headrest speakers are wired with the drivers seat as the front and the passenger seat as the rear speakers. This is due to each seat having a right and left speaker with tweeters. It sounded ok but there was really no bass. I took the 5-1/4" speakers back to Best Buy and bought some 6x9s. I bought some 6x9 speaker boxes and wired the amp to these speakers. The boxes are as small as one can get and were about $35 for the pair. The Pioneer radio with the 6x9s in boxes and seat speakers sound great. The radio's internal amps are wired to the seats and the L/R RCA jacks are going to the amp and then on to the 6x9s. I'm not using the subwoofer jacks. I know there are some purists out there that wouldn't cut the H frame but I don't see it a problem as there are stock H frames out there. I don't see ever going back to a stock radio. I really enjoy the CDs, MP3, thumbdrives plus I can run my Iphone through it. The exhaust note sounds good but when cruising down the road a little road music really makes the drive enjoyable. I installed the amp on a piece of plywood I installed on the passenger side behind the glove box on the horizontal body sheetmetal above the passenger's feet. This is about a 10"x11" area. I used a few angle brackets at the fore and aft end of the board. This way I could attached the board on vertical sheetmetal. Two screws are on the same vertical piece of metal the windshield washer is mounted on and a one screw behind the glove box on the vertical sheet metal. I did this so no water that may collect on the horizontal piece of sheetmetal in the corner of the engine compartment could leak past the screws and into the passenger compartment. I think I'll make some new solid kidney pieces to hide the radio and clean up the trans tunnel. I have the kidney pieces that have 5-1/4" speaker holes in them. They are upholstered in a flat black vinyl. They're homemade, look great and fit well. I used Velcro to attach them to the console but brackets or screws could be used to attach them to the H-frame and a bracket could be make to support the firewall end. They're free, just pay for shipping, maybe $10-$15. I can send pics, just let me know.
Thanks for looking and replying to my post.
Art