Monsoon 6x9 replacements?
I'm also not sure what speaker to go with but you'll want some 2 ways back there and you will have to splice into the new speaker. It's not plug and play. You'll be sending the mid/highs to the mids/highs of the speaker and the lows to the lows.
That or you could bypass the Monsoon amplifier.
I'll get you some of the info that I found online later.
if you have any questions let me know

