A friend just bought this vehicle and asked us to help fix it up. Fix one RV and they think your an expert! Anyhow he took the horn assembly apart and I can find a diagram on how it goes back together. Anyone have an exploded picture of the horn assembly? Also anything with part numbers for the windshield washer assembly, the delay switch isn't working? I'm sure we'll find a lot more that needs fixing but there are manuals for most everything else.
Holiday Rambler just assembled their products from parts purchased from suppliers, just like most RV manufacturers. So, as LVJ58 suggested, try and find out the original manufacturer's name and go from there. If that fails, try a auto/truck parts place, like NAPA, and see if they can match it.
A second choice would be to contact the chassis manufacturer. If the coach is on a Ford chassis, go to a Ford dealership and see if they can identify the part. Almost all of the running gear parts were supplied by the chassis maker.
Holiday Rambler Navigator DP, Hummer, and Honda VT1100C Shadow