Actually the only thing I would fault Uniden on is not including a line out (aka "Record Out") jack on their new series. I'm sure it was a cost cutting measure and they really figured on the SD record feature and maybe the Siren App (if I understand it correctly) or some such for those who need to stream audio. Otherwise, the external speaker output is a good design albeit intended solely for a speaker and not for any interconnection to any other devices. As a external speaker output, it is fine and provides a good amount of power with decent efficiency. It's just not a good audio interface for device-to-device line level usage. But then, most speaker outputs are NOT designed for that. It's just that, in many cases, one has no other option and has the external speaker output as the only audio output besides a headphone output. The GRE 500/600 series radios have never had a line output. Users of those radios have always had to deal with attenuating a power intensive speaker audio level down to a line or mic level when recording or streaming. But they did not have BTL drivers, admittedly, to deal with in the process (so no major ground issues).
Uniden, in a sense, "spoiled" their users by including, very smartly, a nice line output on their 15 and x96 series radios. Unfortunately, again, I'm guessing for cost cutting reasons, they decided to omit this much used feature on the new series forcing Uniden users who were accustomed to using the "Record Out" jack to now resort to using the speaker output or the headphone output as GRE users had been forced to do. In addition, given the BTL design, it caused great confusion among users when grounding one side of the BTL amp caused issues!
A BTL design is a fine choice and much used among professional land mobile radios. But most of those radios also include line outputs (or, even if they don't, most are installed by trained professionals and the installations are, thereafter, not altered by the users anyway). But BTL outputs without protection do not like to have half of their output shorted to ground when some users unknowingly try to plug a simple grounded sleeve/shield plug into them as would be done when trying to use the output for device connections other than a passive speaker.
It's just an unfortunate oversight on Uniden's part complicated by their choice (actually, for speaker usage a GOOD choice!) of external speaker amplifier design.
But you CAN work around it!
If you try my designs, try the simplest one first, the "direct coupled fixed attenuation" version as that will be the easiest to construct and get parts for. I think the transformer versions should only be tried experimentally by those familiar with basic electronics as I didn't specify an exact part for the isolation transformer and that part may need to be experimentally chosen (though I would think just about any audio isolation transformer SHOULD work, in theory, except for autotransformers). If you need variable attenuation and want to run a speaker at the same time then the direct coupled version with variable attenuation should be fine. Feel free to experiment with the attenuation resistor values to see what works best for you - just understand how voltage dividers work. The values aren't extremely critical.
If you only use it for the 536 you can omit the jumper and related parts and wire it strictly for BTL usage. Also, the switch for ground isolation and choices could be omitted if you don't think you need it (you can just dink with the ground directly if you need to - the switch was put there to allow maximum flexibility when using the adapter among and with different devices and setups).
Feel free to PM me directly if you want to ask me anything about the circuit or construction.
-Mike