Behind the headlight assembly and under the head unit are pretty much one in the same.
There is not really any room behind the head light assembly, There is however a lot of room under the head unit. Thats where I installed my Bike Tronics 4180 amp. You can check it out here..
http://harleytechtalk.org/htt/index.php/topic,80359.0.html
Good luck and remember to get your radio flashed..
Jumping in here - (sorry for bad pics)
running ( one Hawg wired amp 2 channel)
1) I unbolted the "boom speaker pod" ( R SIDE FACING FRONT OF THE BIKE)(5 screws I think) there was a lot of room under the black shelf bracket (on both sides L&R of the radio) - used velcro attached the amp on the shelf-
2) bolted back up the speaker pod, amp fits perfect as if that space was made for it - adjustments are easy as well in that location - and the bike looks like it came - (leaving room for more upgrades)
3) as said before there is the exact same room on the other side for another amp of the same size-
4) I velcroed to the roof inside the "boom speaker pod" - the JL Audio C5-650x L & R separate adjustable crossovers behind each speaker
5) Also taped the boom box pod for a tighter sound - some air was escaping
- no problems