I realize the stupidity of the event, and the vagueness of the question so allow me to divulge. Please keep in mind that the last thing I need to hear is what I should have done, seeing how I was not the one to do it.

My brother installed the battery on my 06' Mazda 3 backwards. There were sparks, the horn went off, and then the car cranked (the keys were in my hand!). I was told that the battery's polarity may have been reversed so we exchanged the battery (that was a headache), installed the new battery, and now all the lights come on in the car, but there is just one click (the engine won't even try to crank).

I've checked all of the fuses and they seem to be fine. In my research I've seen a lot of people suggest that it could be the MAIN FUSE which is a fusable link connected to the battery. From what I have read, it can act as a fail safe if there is a jolt of power, protecting the ECU from being burnt up.

Does any of this sound right? Does anyone have any suggestions based on experience (both been there, done that or seen that, fixed that).

How can I test the Main Fuse? Where could I purchase a new one? Please help.