Gary's horn harness uses a 30A NC / 20A NO relay, very common.. can buy it lots of places...
I have trouble believing the relay you bought, had no ratings printed on it...unless it was a 'specific OEM replacement', normal multi use relays (noted as such) do have it marked.. it just doesn't happen, all have a rating.. but hey... I don't have it in hand.
https://www.murphskits.com/product_info.php?cPath=25&products_id=221&osCsid=qkQB4H2LYWebsxMNmwL442
looked at AutoZone, most if not all of them are 20A, and will be fine... (i have one in my garage stuff right now, and it's marked tho, and I got it at A/Z)
glad you got it fixed tho.
Many relays sold as an OEM replacement, do not have the rating printed on them. This one did have the basic diagram for the pin outs though, but no ratings, and none in the box. I know they are common, but I wanted one today, I have plenty of sources for them online and locally if I order, but with the holiday weekend, it would take awhile. I can get the same one Murph's sells from Mouser for $2.54, other places, less than $2. I just wanted one today. This was the only one they had in stock, of any 5 pin 1 Form C relay, so that was my choice.
If it fails, it would just weld itself closed and the horn would sound until I pull over and pull the relay.
I'm sure it will be fine. I doubt it has a resistor like the one from Murph's, but that isn't necessary for this application.