The nav receiver, G/S receiver, and G/S indicator are connected each to the other in a triangular, or three way wiring network.
The glide slope frequencies are already received by your nav radio, but it can't decode them to drive the VOR/GS indicator properly. The G/S receiver frequency is married to the nav radio frequency by an existing output from the nav radio. If a VOR signal is received, the nav radio drives the vertical needle of the indicator in normal VOR fashion, while the horizonatal needle displays "off". The G/S receiver ignores VOR signals. If a G/S signal is detected, the G/S receiver decodes the signal, and sends the more sensitive course correction signal back to the nav radio, which only amplifies the signal and in turn drives the vertical needle normally. The horizontal needle is always driven directly by the G/S receiver.
Most nav/com radios already have the circuitry and pinout for adding a separate G/S receiver, but the radio mounting tray will need to have the wiring harness added to the existing connector. The cannon plug for the indicator will also need to be changed, using existing wiring from the nav/com (vertical signal), plus new wiring from the G/S receiver (horizontal signal).
The Narco 825 puts it all together in one package, but it won't be "plug-n-play", either! You'll need the correct mounting tray, preferably pre-wired with the correct cannon plug to drive the replacement VOR/GS indicator, and a duplexer for the existing nav antenna, which will double as a G/S antenna. On some installations, all of the ships audio passes through the comm radio's audio amplifier, so that's also a consideration.
If you plan to keep the King as nav/com 2, additional audio connections between the two radios and to the comm panel will be needed, and may not be completely compatible between brands. You'll need a second comm antenna and coax, a speaker isolation relay (to prevent feedback), and a triplexer for the existing nav antenna (two navs and one G/S on the same antenna).
The only disadvantage I see to a single Narco 825, is if the internal G/S receiver has problems, you loose everything while the radio is out for repairs. But, it is lighter than my system, and requires less installation of components and harnesses. If you can afford it, have the panel space, and the radios are compatable, I'd move the King to #2 position, and install the 825 in #1. Labor cost may rival radio and components cost, but it does pay to shop around!
