Building a Bluetooth speaker

The BT receiver and decoder Stands on a thumb..or a stamp. Needs 5V supply or less (the 3.7 V potential provided by a lithyum Cell)
Just attach It to anything that has line-in and two channels.
If the sounds at start up are annoying Just provide a switch to mute
 
If noise at start up Is problem and a volume potentiometer Is not in the circuit, a switch might be substitited by a relay, so It can be 'programmed' to open contacts (or close them: if you put It at line level and use It to put to ground the signal) for the necessary time
 
and preferably cheap.

Probably cheaper will be to buy Bluetooth speaker at Aliexpress.
Unless you want to gain experience in designing, soldering, drilling, sawing, planing, painting.

The BT receiver and decoder

IMHO, the electronics will be the easiest and not the most expensive part of this project.
The cabinet and other mechanics will be more difficult.
 
I'd simply take something like this
BTradiocharger.jpg

It has more features than you'd want, but I find 'em all useful.
The cigar-lighter pod can be milled, anyway...
...you have the access to the 12-16 V terminals and if, as I think, the thing is mobile, it should be running with batteries.

Edit- whoops! it has no line out jack! It just connects via FM radio, as it's made for cars without 'wired' input