Disclaimer: I am a noob to car audio and the like.

Hello,
I purchased a 2005 Limited Coupe in late December. At the time of purchase I was aware that the sound system in the car didn't work. It had an aftermarket pioneer head unit and the factory amplifier. I knew of a decent car audio place nearby my house and took it there to get it checked out. They took about an hour to look at the car and came to the conclusion that ALL of the speakers were blown, so I had them replace the speakers. To no avail, that didn't work either. (They were also convinced that the car had only 4 speakers in it, the woofers and tweeters. ) They eventually somewhat narrowed it down to my amplifier. They found out that it was blowing a fuse somewhere. However, I am concerned that there is a bigger issue. I have come to notice that my power seats only work part of the time. I can't tell a pattern to when it comes and goes and it seems totally random. That being said, I have a few questions.

1. How can I test if the amp is the actual problem? / Should I purchase a used OEM amp?
2. Can I purchase an aftermarket amp and use the factory wiring for the factory speakers?
3. Did I purchase a dud with the seemingly random use of the power seats?

Also, sorry for the formatting. I am really bad at writing about stuff i'm not really knowledgeable about. Makes my brain scramble around and randomly write what I am thinking. Cheers.