I tend to listen -1 bass, 0 treble, audio pilot on, and surround -6.....The problem with the surround, is that if you turn it up, you get a very wide soundstage (it makes the car feel 20 ft wide), but you loose all the vocal imaging, and since the car has a center fill speaker, it's a shame to do that.
Here's the way I do it
Find a well recorded cd, with music that has a full dynamic range without overblown bass.....so put away Kayne West Goldigger 'cos it ain't going to work with that! Any of these will do, Annie Lennox, Fleetwood Mac, Coldplay, Seal, Keith Urban, Santana etc.
Make everything neutral, no audio pilot, no suround etc. Close your eyes and you should be able to hear the main vocal singer sitting in the center of your dash, switch on audiopilot, and surround, and turn the surround all the way to the min setting (minus whatever)....very slowly increase the surround setting to the last point where you can still visualise the singer's vocals in the center of the dash. This will give you max soundstage whilst still maintaining correct imaging.....I really find it helpfull to close your eyes and point to where you hear the vocals and instruments etc.....If you go too far it'll be like 69bossnine said 'muddled' you won't be able to tell where the vocals are, they'll seem like they are everywhere.
Adjusting bass and treble, is a lot more subjective...The best approach is to make everything as smooth as possible, If you apply too much bass, you'll loose the mid bass and the sound will be bright vocals and heavy thud with nothing much in the middle, the detail of the music will dissapear. Apply too much treble and you'll introduce listening fatigue...you'll want to turn the music off, and if you're slightly tired it'll give you a headache...a smooth balance is best. Final tip regarding the bass...make it a little stronger than balanced when you initially set up (with engine off/stationary) because when you drive, the road noise/engine will remove the extra bass you dialed in.
To adjust balance find a track with a classical (non electric guitar) solo....It has too be a single guitar....If your balance is out, it'll sound like there are 2 guitars (due to the different time it takes for the sound from the left & right speaker to reach you) the dsp should compensate for your 'unnatural' (too far left) listening position and compensate for it, so it should need minimal adjustment.
Fader adjustment is also a balance, too far back and the music will be 'pushed' from behind you, too far forward and the music will sound 'thin'.
You'll have to go back and do the entire above procedure a couple of times as the controls inter-relate to one another.....
Even after you've done all this you'll still have to make adjustments on individual cd's to compensate for differences in source material, but this will give you the 'best fit' for most music
BTW don't spend too long at any one time trying to do the above, because your ears adjust and you won't be able to correctly distinguish the various adjustments, also don't try setting up with the music too loud as this has an impact on your ears' ability to 'finely' tune
Sorry for the long post...but you did ask!