I have been struggling with a situation similar to this. My '85 K5 came cut with an Alpine CD player in it that spits out CD's. It won't play CDR's or regular CD's, either. I was going to buy an old Pioneer Super Tuner cassette deck and have an old electronics repairman I know clean the head, align the tuner, and replace the belts. For digital music I was going to get one of those FM transmitters that plugs into your cig lighter that hooks to Bluetooth. Then I could broadcast music from my phone into the FM transmitter via Bluetooth and into the tape deck via the FM transmitter. Sounds a lot more confusing than it actually is. I even had the specific model of tape deck I wanted to get picked out in my mind - a Pioneer KP-500. These things are badass. I actually restored one about 10 years ago. I bought it in a pawn shop for five bucks, found new belts online for another five bucks, and had my stereo guy repair it for $15. I had a grand total of $25 in the thing and ended up selling it on eBay to a guy in Australia who was going to put it in a '79 Trans-AM he was restoring. I got $700 for the thing. Looking back, I should have kept it because they still come up on eBay from time to time and sell from anywhere from $200 (needs repair) - $500 (NOS). I was seriously thinking about getting one again and running the phone>>>FM Transmitter>>>tape deck setup, but I just couldn't bring myself to drop $500 on old stereo tech - which sucks because these things are so damn cool. The analog stereo dial is round like a clock face instead of linear like most analog tuners and the dial lights up orange and the numbers light up this really cool aqua blue color. The face is brushed aluminum. I can't think of a head unit cooler than these, modern or not. Here's what they look like:
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Anyway, since my Blazer has already been cut (I do have 2X extra dash bezels, though) I got a Pioneer Bluetooth (no CD) head unit from Walmart for $50. I bought two 3.5" speakers for the front to go under the dash pad and two 4"X10" for the very back for a little over $50 new at my local pawn shop. They are those cheap BOSS Audio speakers. I also bought an Alpine amp model MVR-F300 to push them that will be going underneath the passenger seat.
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My Blazer is just too far from "original condition" to worry about cutting a stereo hole now. It's had a color change to black, had the 305 and smog stuff yanked out in favor of a 350 and straight pipes, and the 10 bolt rear end that came with it swapped out for a 12 bolt from an '81 Suburban.
I would have preferred an all original truck, but since it's already been messed with it kinda gives you a "blank canvas" to work with and do some stuff you wouldn't normally do to an all-original ride.