Modify an outdated radio to connect to Bluetooth and hear music from a phone or computer.
Step 1: Take It Apart
Unscrew the back of the boombox so that the inside of the cassette deck is accessible.
Step 2: Prepare Bluetooth
Remove the case of a cheap Bluetooth adapter so that wires can be attached to left speaker and right speaker outputs, and ground. Solder these wires to the adapter (I used a black wire for the ground).
Step 3: Connect Bluetooth to Tape Deck
De-solder the left, right, and ground wires connecting the boombox to the cassette deck (on my device, they were soldered to the cassette reader near the bottom of the deck), and solder each of these wires to those connecting to the Bluetooth adapter.
Step 4: Finishing Touches
I fastened the Bluetooth adapter to the inside of the cassette deck where it would be accessible when the boombox is put back together and the user opens the deck. Reassemble the boombox, being sure that any of the new wires do not hinder it from closing. If the speakers are not already attached, hook them up to the boombox.