skeetbum e-mailed me about some of this. He may have to contact the Humminbird Customer Service folks about the 10-foot cable. If all depends on if he only wants the 10-foot cable (also used as the AS-EC-10), the 20-foot cable or the shorter cables that have the trolling motor interference chokes built into them. If it’s just the 10-foot cable he can get that at any retailer that carries the AS-EC-10 [NOTE: this is a different cable than the closely named AS-EC-10E] but if he needs/wants the others he will have to deal with the Humminbird Customer Service folks directly.

For the GPS: Rickie is right in that one GPS Receiver (either the internal or external) can be used by both units but you will get more accurate waypoints if each uses its own GPS Receiver and preferably it is mounted directly over the transducer or as close as you can get to it. You can set the units up this way by selecting which GPS Receiver they use in the GPS Receiver Override menu.
If you only want to use one GPS Receiver you have two options here:
1. Use the internal GPS Receiver of your 798ci Si unit. No extra cables to connect and you will have the external GPS as a back-up or to sell.
2. Use the external GPS Receiver of the 798c Si unit and plug it into the GPS port of the InterLink. By default; both units will use this external GPS Receiver. As a backup you can select for both units to use the internal GPS Receiver of the 798ci Si unit but it does have to be on for the other unit to use it.