Hey Bob,
for the horn you will really want to consider the Stebel Nautilus. It's inexensive and LOUD!!!
The normal installation on a Std Roc is under the left side cover but I don't know if there is an open space there on the R3T. You can find well documented installations here or on the other site (.com).
btw, I just got off the phone with one of my vendors down in Corvallis (sp?) and they were talking about the snow down to 1000ft this morning. Where's spring?