I recently installed the Polk DB691 (front) and DB651 (rear) in my Frontier.
Yes, Crutchfield says they wont fit, B.S!
The only thing you will need to do it trim the backside of the factory plastic adapters where the magnet is larger in diameter and trim off the bottom lip where the speaker sits. You will trim off a small half moon size for the magnet and remove the lip near screw holes. Once you do this it will bolt right in without spacers or 3rd party adapters. It took me 10 minutes to do both doors and that included removing door panels.
You have enough room in the speaker harness (at speaker) to cut and terminate new leads to fit onto new speakers.
blue, red stripe = negative
blue, white stripe = positive
solid blue = negative
white, blue stripe = positive
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