Yup. While not exactly mounted in a car, while i'm hitting the trails on my mtb, omp in car mode and modrana are by far the best solution i have found.

Its really useful to download maps of ~50m (or km's for logical units of mesurement, but hey im a merican' :P ), when you use a prepaid tmobile service, and once you get out in certain places theres no service at all anyway. Just a bit of wifi downloading 30 before my ride and im set.

OMP, just recently downloaded, and havent used the stock media player since. Faster, more features, the car view is just awesome for biking whether for transit or fun/exercise, and if you want to shuffle your entire library (8000+ and growing in my case, 64GB sdxc microsd, 32ish GB onboard, really the only data i have on my n900 save for app data, modrana maps, and the odd tv videos i put on there), it just requires making a playlist, though it takes a good amount of time, in my case ~1hr to process.

Then the icing on the cake, was when i clicked a normally choppy flash video one night in the stock browser, and OMP popped up, streamed it flawlessly with no seek issues as i find alot of other streaming video attempts fail in on my n900. Prettty frickin sweet compared to inbrowser flash.

Just have to switch windows from modrana/omp, prob an easier way to do that than the stock method too, havent looked into it yet. Not super duper lazy, but i reserve the right to do so lol.

And forget BT, hardwire it into a phono input if you can, BT is decent, but audio quality suffers if you listen to anything above 128kbt mb3's. If you really want BT, get a decent reciever, i've been mulling over one of these bad boy's recently, looks like a very high quality board. Just have to do a bit of diy wiring, and put it in a project box of some sort to get it setup. I dig you can choose a li-ion battery of your mah preference, and it has a built in charger. Only $40 too.

The diy junkies dream lol, i'm holding off til the other handful of projects i'm working on are completed though. I'd wager its a far cry better than the meeelectronics airfi af1 i have though. And you can use an external dac, since in reciever mode (one board, can work as either a transmitter/reciever/multitude of functions/options, pretty configurable) it can kick out 48khz from a digital input (prettty sure wireless BT would fit that bill, but n900's ceiling is prob going to limit that to 44.1khz).

Still, future proof with usb 3.0, aptx support, highly configurable, and the price is incredible for what you can do with that board, a bit of soldering, and diy work. If we ever figure out how to get a bt transmitter via usb working w/ the n900, and can do aptx 48khz, by god it would be magnificent. Or for future devices/current devices that can do that right now. Analog wired would take a back seat as far as the quality advantage in most cases.

http://www.kcwirefree.com/audio.html

The one i'm describing-

http://www.kcwirefree.com/kc6112.html

It would make good use of my broken n810's 1500mah lipo, very good use indeed