My 754 does not have a black box on the back. The back of my 754 looks like the one in this auction except on my 754, the little white plug near the antenna lead on the 780 is non-existant, and the 754 I have has a recessed antenna connection instead of the external cable as shown in the 780 auction. The power, speaker, ground, and antenna connections are the same in both.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&ih=013&sspagename=STRK%3AMEWA%3AIT&viewitem=&item=230159166996&rd=1