In the wife's Mazda, I put in some
Dayton Audio ND25FN-4's. They fit almost perfectly in the factory locations. A small dab of glue from a hot glue gun holds them in perfectly.
Only $15 each, and for that tiny bit of money, they sound damned good. In her van, they are running active, so they get their own dedicated amplifier channels. The crossover I'm using is the
Dayton Audio DSP-408 digital signal processor. If you're not running active, you could easily get away with whatever crossover your tweeters come with, or failing that, a simple $2 cap would suffice. If running a cap as a crossover, set the point pretty high, say 5,000Hz, because a cap only has a roll off of 6dB per octave. (You can look up what that means if you're not sure. Crossover theory is a rabbit hole that is way beyond the scope of this post.)
For what it's worth, her mids are
Dayton Audio RS180-4 7" drivers. They sound absolutely wonderful! Far better than what their low price would have you belive. They fit in her 6x8 factory openings using a 6x8-6.5" adapter plate. I had to slightly modify the edge of the basket by trimming it a tiny bit so the door panel would go back on. Your mileage may vary.
Before that I had some D
ayton DC160-4's, but I had the crossover set wrong, and the wife got a little crazy with the Missy Elliot, and one of them went poof! The DC's fit with no modification at all, and sounded pretty damned good, too. Not going to lie, the RS's do sound better, but honestly, both are a steal at the prices they are.
pic: Dayton tweeter vs factory junker in the door sail panel cover
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