I'm guessing this reply may come too late but I'll take a look when I get home and try to send a pic
Sent from my SM-G960U1 using Tapatalk
Thank you @spearoid
. I couldn't find the relay, but I found a fuse labelled as BOSE amp in the main box under the bonnet. Remarkably, it was the culprit, at least in my case. While looking for the relay I found the fuse and noticed the whole box was very dusty. I realised the contacts might have corroded, so I decided to clean them. Since all other electronics on the car worked well, I chose to clean just the amp fuse and its socket. And it worked!
Cloth and contact spray did it. That's all.
The amp didn't start right away, but next morning it turned on and has worked without a fault for a few months now.
In short and for the posterity, BOSE amplified intermittent operation (not working sometimes) problem could possibly be solved by cleaning pins of the amp fuse and the socket in the fuse box under the bonnet.
Please don't forget to disconnect the battery while doing this and good luck.