Bosts if you can't find what you need you might consider just buying a separate unit. For $180 you can get a 160c. The black box you are looking at isn't much less from what I can tell ($177). Here is the 160c I found (shipping included in the $180):
http://cgi.ebay.com/GARMIN-FISHFINDE...QQcmdZViewItem
I understand why you are looking at doing it ($$) but there are 2 disadvantages to having the combined unit IMO. One is now you are sharing your already small screen between 2 functions. Two is if the unit malfunctions, now you've lost both the sounder and chartplotter.
Blue's .02