Any advice on stealth mounting of XM and GPS antenna would be appreciated. I was thinking
1) Inside Windshield behind rearview mirror for GPS and XM
2) Under Vent Cowls on Hood
3) Behind the Front Grill
What are the downfalls of these
Has anyone attempted these? I have a 2007 Mercedes C-Class and I would really like to not have any visible antenns 
There is another really good thread on this topic.
I have both antennas under my dashboard and they work fine.
windshield behind the mirror is probably ok as long as they are not obstructed too much,
I would avoid behind the front grill, you want them to have an open view of the sky, metal from the engine compartment would block signal.
under the cowl is good if it is a plastic cowl, bit of a job running the wire but all in all a good location as long as there is no interference from windshield wipers.
the wire going to these antennas is a mini coax, so you have to be careful not to pinch it.
Personally, I would recommend temporarily trying the mirror location first, just tape them in place for a few days to see how they perform, if ok, then permanently mount them.
Search the forum, I think this topic is covered very well.
http://avic411.com/forum/viewtopic.php?t=11535
good luck
Your setup looks great. I have the perfect spot and on my Mercedes. There is a huge Center Channel Speaker in the middle of my dash that is not utilized because I did not go with the upgraded audio.
How is your reception on GPS and XM? My positioning will be low but the antenna's will be exposed through the plastic grill. How come everyone says you need to put them on the outside. I see more and more people mounting inside car?
Nice install 
There is another really good thread on this topic.
I have both antennas under my dashboard and they work fine.
windshield behind the mirror is probably ok as long as they are not obstructed too much,
I would avoid behind the front grill, you want them to have an open view of the sky, metal from the engine compartment would block signal.
under the cowl is good if it is a plastic cowl, bit of a job running the wire but all in all a good location as long as there is no interference from windshield wipers.
the wire going to these antennas is a mini coax, so you have to be careful not to pinch it.
Personally, I would recommend temporarily trying the mirror location first, just tape them in place for a few days to see how they perform, if ok, then permanently mount them.
Search the forum, I think this topic is covered very well.
http://avic411.com/forum/viewtopic.php?t=11535
good luck
Just put it underneath the dashboard.
Thank you, it was quite a bit of work, but I am finally happy with it.
Sorry I did not take photos during the install, if you look in this photo below, I have both antennas under that top plastic panel just above the radio. I havent had much trouble with reception, but I never had them mounted on the roof so I cannot really compare, but for me, they both work fine under the dash. I previously had an alpine system before I discovered the pioneer avic d3 and I had no problems with the antennas there either. I had an experienced installer advising me and it was his recommendation to go under the dash. On older gps systems, I think external mounting may have been necessary but the newer systems seem to work fine inside.
more photos of the rest of my stereo here: