Here's a
link to a thread I started on this topic at that other site...
In the end I used a DPDT switch - the BT had to be off while the radio was playing AM, otherwise it produced interference.
Whichever way you go, use an isolation/auto transformer. In the BT example, a
1:1 was perfect. If you add an aux-in, you'll need a
1:4. Oh, and remember to mix the stereo to mono with a couple of 2k resistors (or thereabouts).
The audio out of the BT adapter (or the terminals of the aux in), via the isolation transformer, connect across the volume control of the radio.
Personally I prefer an embedded BT adapter to an aux-in. No faffing about with a separate dongle.