well I have grom audio. Make sure you are getting a second generation as first generation mixes audio together. meaning in aux mode you would still hear USB playing. Okay with my grom I have usb, ipod and aux. plus I have the oem aux.
issues if you do the Bluetooth with grom then the factory Bluetooth becomes useless.
but they open the ability to have Bluetooth streaming. I went another route for that. Finally if you don't have an ipod. there is an ipod to aux adapter available on the web so then you could have 2 aux ins. The usb is limited to I think 2,000 songs for random play and it has killed 2 wd hardrives of mine in a 4 year period I had it. Some of my songs it glitched on but they do provide fixes and patches but for now I just happy with Bluetooth streaming.

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E53 06 3.0i

wishlist
Comfort Seats
speckdock 2.5

Mods
Air suspension - wip
skid plate replica - in box
OEM sunroof deflector
OEM bluetooth streaming
navi,dsp,tv retrofit - wip
Low prof roof racks
2 directtv satelite tuners
Grom audio usb2
quarter and back window sunshades
2 10"subs- boxes opposite sides/Alp MRP-M850
rear heated seats retrofit
rear footwell lights retrofit
remote window roll up