Hi John, we have the 2004 36' Heritage model and we have the horns on top as well. When we bought it, there were several switches that didn't have any labels on them. One of those switches was located on the dash, right hand side. SHould say this switch doesn't look like the other switched, this is just a toggle switch. Asked the previous owner, we purchased from a dealer, what that switch was. He smiled, told me to hit the horn button, I did, normal horn, he told me to flip the switch and hit the horn button again, AIR HORNS kicked in. There is a air compressor with a holding tank on the drivers side outside under the "hood".
The other switch is located in the power compartment. He had no idea what that was. Checking here on irv2, found out that is the 30/50 amp switch which you switch to what shore amperage you have. This is for the air conditioner. As I understand it, on 30 amp, only one compressor comes on, on 50 amp both will come on as needed. This might explain why he had the a/c system replaced about 8 months before he sold it. It was in the 30 amp, down position.
If your horns are 12v, might check for a fuse.
Ed & Jan, USMC Ret. 67-87, Vista, Ca.
'04 Dolphin LX Heritage, '2019 Frontier tow
Ham Radio, USCGAux(comms)