I'm betting some of you have heard this, and experienced this. For me, it started 4 years ago when I got my '05 Boxster. The battery was weak and subsequently died as it took over a month to get the car needed maintenance. When I added the new battery and turned on the ignition the radio system produced a god-awful screech through the speakers like the amp went full volume and shorted all inputs. Scaring the crap outta me, I shut it off.
I learned later this was the main power amp in the trunk acting up. Each winter, I'd unplug it, plug it in in Spring, and I'd get the screech as it activated.
It only did this once a season.
A month ago, I came across a DIY amp re-cap. This was quite extensive, re-capping ALL caps, including the SMD devices. As an electronics guy, I was intrigued.
I got all of the caps, and ordered them for around $50.
I took the amp apart, (fairly easy, but there are 2 firm ribbon cables to be aware of) de-soldered the main power supply caps. (Not terribly easy-desoldering)


Amp opened

Replaced the 5 main largest caps, re-did the thermal paste on the output transistors, and buttoned it up.


These 5 caps

Yesterday was coming-out day. Finally decent weather here to work on the Boxster in the garage. Yes, a new battery, lasted just 3 years. Put the amp in, and checked all connections.
Turned on the ignition...The Radio system came on with no screeching.

I have details for you if any wish to try this and a list and values of the other caps and where to get them. From what I've seen and read, it;'s the power sully caps that go bad. At least it was for me. Saved $400.