"most likely they're just blown and need to be replaced..."
Seconded. It won't be the head unit as then the mids wouldn't work either. Check the cross overs (if applicable) but most likely it's time to pull them and replace.
Also though, did Malibu come stock with Polk? I thought they have used Sony, Kicker and Rockford in the past but I was never aware of an affiliation with polk...
My advice would be to hit up wetsounds.com and try out a pair of their 6.5" drivers, otherwise you'd be sure to find a pair of the polks on Ebay as a cheap, direct replacement.