https://www.youtube.com/watch?v=HHJxGh8kaT8
There you go. There is a schematic in the vid, hopefully it's of some help to you. And actually I consider the bluetooth a much safer option than a aux input jack. I mean, all I did was add an aux jack to my radio, and plugged in a usb bluetooth receiver, and it didn't ruin it. But my thinking is, it'd be safer to risk a $7 Chinese bluetooth thingy than something potentially much more expensive like a smartphone. I suppose you could just add the aux port and instruct the user to exclusively use a bluetooth receiver, and nothing else.
Yes, there is a variation in volume from the radio and the BT. However, that could be tuned by changing the (in my case) 10k resistor and also the two stereo-to-mono resistors.
Also, looking back, I just remembered that the aux input's ground is directly connected to the 120vac Line. So conceivably somebody could get zapped if they touched something on house ground and the metal jack at the same time. Not that it would be harmful, just very surprising (read: 'shocking'). I suppose you could use and audio isolation transformer if you had to. But if you keep everything inside the radio, it'll be plenty safe.
https://www.ebay.com/sch/i.html?_fro...tooth+receiver