I was going to ask the same question. I know UNIDEN scanners like the 996t and 996XT you can set all systems to RECORD and use the REC OUT jack. This puts a LINE OUT feed to RR and VOL Knob doesn't bother the level. The GRE I have no clue and I am ALL UNIDEN here. Someone else will have to take the ball and run with that one.
You can use the ScannerCast program to send audio/tags to RR.
On your volume mixer, under Properties, then Playback, there should be some check boxes for Microphone and Line In. Make sure whichever one you are physically plugged into on the soundcard is checked. Then go back into the mixer control and make sure that input is not muted. Should be able to hear your feed just fine locally.
I have ScannerCast running here locally, I am just not broadcasting to RR (yet). The scanners are on a totally separate computer from the computers I work at. I use WinAmp to listen and "play URL" using the IP address on my LAN. There is a delay between broadcasting and listening, but if you are sending tags you should be able to see them this way.
If you use a scanner with a line or record out (Uniden 15, 996 or other), two cables are all you need where the "Y" is connected to the scanner record out.
If you are using a scanner that does not have a line or record output, just connect the "Y" to the scanner headphone out and add an external powered speaker to listen with.