In-dash EQs, as well as HU EQs, are kinda worthless unless you're looking for dancing lights and knobs to turn.
The 1 thing you should look for in an EQ is the ability to EQ each channel (left and right) separately. I do not know of an in-dash unit that can do that. You want this feature b/c the interior of your car is not perfectly symmetrical and you use the EQ to cut out peaks that occur b/c of environmental reasons & the usual bump you get at your crossover points. Furthermore, you'll need an EQ tone down the natural peak of the speaker. All speakers have a natural resonance and they're most efficient at this frequency. What happens, usually, is this is also where they're the loudest and a peaky mid will actually sound kinda harsh. EQ that SOB and you're done.
But if you're going to get into an EQ that can do all that, might as well get a real crossover & EQ combo. The crossovers on most amps really do a poor job and can dirty the sound. In addition, you have to trust the silk screening done by the nice Chinaman to know where you're crossing things over. This isn't a big deal with subs, but, if you hit the 10x switch and you have to guess where the crossover point is...well...getting it wrong will smoke your tweet. Use the x-over in your HU (if you have that feature) and throw a cap in-line just in case your system pops or resets.
Now's the part of the episode you'll want time alignment...which is a very slick feature. And you'll start looking at higher end processors that can do everything. But here's the funny thing; none can adjust for phase. Guess what; higher end passive crossovers can and can ball-park time alignment thru this phasing. It's why some component sets cost an arm & a leg; drivers are cheap...it's the passive that eats up most of the man hours in R&D.
But what do I know? Wenn's the expert and his expertise is so top-secret you have to PM it. Disregard everything I wrote and look to wenn for audio salvation. After all, I don't even have an avatar...so what do I know?