OK The speaker fires out of the defroster vent.The back of the speaker is taped off to stop the rear sound waves from canceling the front.I don't think this will work too well with a woofer because with bass frequencys the cone moves too much. The taped speaker basket will restrict cone movement resulting in bad sound. I don't know if you've ever had your dash pad off, but I don't think theres too much room under there to hide a crossover. You would have to hide it under the dash and run seprate mid&tweeter wires to the speaker.
If I did this file uploader correct heres the pics.
3.5" coax - I used one with four mounting tabs. two of the tabs extend over the duct work to mount the speaker. I did this 5 years ago so I don't remember what there screwed into, you might have to fab somthing up.
Here's the speaker mounted in the duct work(the black rectangular area that the speaker is sitting in the center of. The duct is soft plastic, I think I used tin snips to trim it. The magnet is actually sitting in the duct. You would have to trim for the particular speaker you use . As you can see (thats a 3.5") there's not alot of room for anything bigger.Don't go too large on the magnet either, my magnet is about 2.5" , try not to go bigger then that.
Here's the finished product with the grill installed.
Almost forgot, the heat has no adverse effects on the speaker.
Hope this helps!
