Bass fades in and out of my after market subwoofer addition,when cracking up the volume, then eventually the sound from my rear speakers go out all together.

I added an aftermarket subwoofer and amp to my
06 Cadillac dts. At first it sounded great. I later
changed amplifiers, for  dependability reasons not
more power. It at first also worked great. About a
month later I was showing it off and turned it up to
max. Later on that night I turned it up again and
about half way up the bass wasn't right. It kept
fluttering in and out. More on certain frequencies
than others. I quickly turned it back down. Then
one day after trying to figure out what the problem
was I cranked it up and left it and  about a minute it
to it all the speakers in the rear stopped working. I
shut the car off and turned back on and they
started working again.
So I went to my bros for a 2nd opinion and we
noticed that the trunk cargo lights would flicker
when I turned the volume up. We decided to
disconnect the amp and turn it up and my problem
stopped. My volt meter on the car reads 14.9 then
gradually drops to 12.6 after driving for a while.
Can somebody please help me with this, it's driving
me crazy. Thanks