Not sure what year blazer you have but there was a recall on the windshield wiper motor on I think it was 95-97 but to be sure you can go to:
http://www.alldata.com/recalls/index.html
for recalls on your vehicle and see if some of the problems you are having are covered under a recall. The power mirror switch on my 96 is a little screwy and from what I hear, many of them are. Your speaker problem could be a loose connection or it could be blown- have to take the door pad off either way.
Good Luck