The link above is for a bluetooth receiver though it does say transmitter. Some can be used as either but not that one. http/www.avantree.com/bluetooth-audio-transmitter-avantree-priva.html
You would connect that to the tape record out on the back of the stereo. You would need a cable with 2 RCA plugs at the stereo end and a 3.5mm mini plug at the transmitter end.
Plan B would be to use a 1/4" male to 3.5mm male cable and use the headphone output.
Some allow you to pair more than one device which might be useful.
If your old stereo has only "IN", you can't use it for "OUT" to Bluetooth transmitter to be paired with your headphones. It might have headphones-out, though - check the manual / specs / the units itself.