Well switched polarity won't cause your speakers to blow. Switched polarity only causes a phase shift in the sound (sounds like one speaker is out of whack with the other 3).
Speakers blow because of two reasons. One is that they are trying to reproduce sounds that they physically cannot (ie. a 4" mid trying to make 60 hz bass or woofers trying to do high frequencies). Or the power being suppied to the speakers is well beyond their rating.
There is a test to determine polarity. you will need a 9v battery and maybe some wires with alligator clips on both ends. Clip the negative side of the battery to one terminal and then touch the positve battery lead to the other terminal. If the cone of the speaker moves outward then then you have the right polarity. Just connect the + and - speaker leads to the same terminal that had the + and - of the battery. If the cone moves inward (toward the magnet) then switch that leads.
Thats the hard way to do it. The easy was is to get an adapter for aftermarket decks that can clip into the stock GM wiring harness. These adapters are color coded and labled to make installations easy.
or just look up the factory color codes for your car on the12volt.com instead of putting DC voltage across your speakers and possibly damaging the coils.