So, when I picked up my '84 GL, I noticed it had the stock AM/FM stereo. So, what to do....
FYI - You truly don't need to upgrade your unit if the stock stereo & speakers work fine.
Let me explain.........
Any 1 DIN NAV worth a damn costs over $500. All of that is a pain in the rear to install. I have a $15 solution that requires no installation, and uses the cell phone you already use (which is hopefully either a newer Android or I-phone). In my case, I have a LG Fiesta 2 - which uses the Verizon network (the phone cost me $60 last year) and this phone allows for bluetooth audio streaming... which is the key. So, you already have a smartphone, this only adds to its capabilities and costs you nothing additionally to use.
Here is the solution. Get a cell phone holder for your car vent (or the $5.88 Walmart cell phone holder that sticks to the windshield). Then get one of the below Bluetooth/FM Transmitter/USB devices (it plugs into your car's power connection/cigarette lighter). You then take your Smartphone, plug it into the USB charging port - connect your phone via bluetooth to the device's bluetooth. It will then broadcast your bluetooth audio stream to whatever FM station you decide the device should be broadcasting at. I usually pick 89.9 (as it usually doesn't have anything being broadcast locally that prevents the usage of that frequency).
I am then able to listen to Pandora or Spotify, listen/look at google maps, and receive/end calls. They are integrated with each other on the smart phone already, so if you get a call... Pandora and Google Maps go silent while you take your call. When the call ends, Pandora and Google Maps audio stream then returns automagically. I even played SlingTV (ssshhhhhhhhh) and my son watched the movie "Guardans of the Galaxy" in the middle of nowhere Idaho in Hi-Definition on the smartphone. And we listened to it in stereo on the stock AM/FM-stereo.
This works so well, I am putting it in all of my vehicles. I used this setup in my 1984 Subaru GL Wagon last week (and my cell phone integrated nicely with the stock AM/FM radio)... and multiple people asked me about it during the drive cross country. As such, we no longer need to buy expensive NAV units or rip apart dashboards. Or if you have a bad stock stereo, just get a 1 din stereo that is flat, has USB and it will fit in the stereo's limited space (there is no need for DVD or CD's anymore as music can be played via streaming or you can use a SD Card to store music).
Folks - we already buy cell phones, why not use them as our car's infotainment device as well? As a matter of fact, I don't use the uconnect NAV on my 2013 Chrysler 200 Limited (as I am tired of updating the maps for $170 yearly), I just use Google Maps on my cell (Google Maps is updated daily), & I tie into the 200's stereo directly into it's bluetooth connection.
So, there are 3 for sale at Walmart (or also at Walmart.com) for less than $15.00. For your case, I'd go with the generic device I posted below (that is selling for $14.97) as it also allows you to start or end calls without touching your phone). I personally use the Monster device & also the cell phone holder in mine costs $5.88 at Walmart & sticks excellent against the windshield.
I hope this makes sense to you. Just make sure you have a cell phone data plan that is 2 GB or more
https://www.walmart.com/ip/Bluetooth-V4-2-FM-Transmitter-for-Car-Wireless-Bluetooth-FM-Radio-Adapter-2-Ports-USB-Car-Charger-with-Hands-Free-Calling/621185790?athcpid=621185790&athpgid=athenaItemPage&athcgid=null&athznid=PWVAV&athieid=v0&athstid=CS020&athguid=4f2c6ad3-163-16e156f1def069&athancid=null&athena=true
https://www.walmart.com/ip/Monster-Inc-Mn-Fm-Bluetooth-Transmit/925473573
https://www.walmart.com/ip/Monster-Gooseneck-FM-Transmitter-with-LCD-Screen/598089849