Since i have only seen double din sat nav units, i am going to assume thats what you are using.
If you have an e46, chances are you are probably going to have to put your Factory HU either A)where your ashtray is (it may not even fit there...) or where you have a 'sunglasses slot'. Your Climate controls will also need to be moved to accomodate the double din. These can go in one of the two places i said for the HU. I know for sure they can go where the sunglasses pouch is, but i dont know about the ashtray, the wires may be too short.
Basically, if this is an E46 job, you will A) most likely have to remove your factory HU, B ) Get the OEM sat nav becuz almost all aftermarket ones are too deep and you will end up having to wreck your ventilation system to get them in there. http://E46fanatics.com
has a LOT of posts in regards to installing sat nav systems in the e46, and from what i have read, you are looking at quite an ordeal. One of their members runs this site http://www.srcstc.com/
and sells a sat nav system that fits.
If you arent in an e46, ignore my post entirely :lol: