The ISO connector carries the power/ground lines, remote out (amp on), telephone mute, dimmer etc. but no audio signals. On HUs that have speaker level outputs and ISO connector(s), the speaker signals normally are in a second connector (or the two are joined for on bigger connector).

The audio signals from the empeg in the car are through the 4 RCA output connectors on the sled (there are also 2 RCAs for AUX in). So yes, you need to hook up the RCAs from the sled to the amp(s).

/Michael