Horn relay question

I have been collecting very old 12 volt truck horn, restoring them and now I want to install a set on my s-type, Yukon and GMC PU.They sound great and will get any idiots attention.  I imagine I cannot and have not tried to hook direct to stock horn lines.  Can I use a relay from the stock horn wireing at the horn and if so what kind of relay do I need.  These horns are fifty's semi horns and draw a bunch of juice.  Got them all on Ebay for under twelve bucks a set and do they sound nice........Thanks

just a normal 30/40 spdt relay should work.

if it's a positive output from the horn button

85 - ground

86 - horn button

30 - battery

87 - horn 12v wire

if it's a negative

85 - battery

86 - horn button

have fun and think about the neighbors...laugh