I've earned an old amplifier with speakers that a friend was throwing away and since the amplifier was not working, I decided to recycle the speakers with an wireless bluetooth set.
Step 1: Check Your Speakers and Choose Your Audio Amp Board
there is a variety of audio amp boards you can get.
i got this one but you can pick an appropriated one for your project.
in my next build i'll try this one for 3W speakers:
it's up to you or the needs of your project. just check some reviews first, the one i got didn't had an AUX line in...
Step 2: Choose and Check a Power Supply
check your amp board schematics for the power supply info.
in my case its 8-25V DC. and i have a box full of old ones.
always check the Voltage to see if it's working well
Step 3: Design a Fancy Enclosure for Your New Amp. System!
Measure the PCB and design 3D file in the CAD software of your choice.
ex.: solid works, fusion 360, sketch up, rhino, blender, etc....