where is the horn relay on a 65 pontiac tempest

not sure

You need to get the repair manual for that car. Libraries should have giant books that cover all pontiacs for 65.

Behind the headlights, here is the wiring diagram, just zoom in on it ,

http://pontiacwiring.0catch.com/Pontiac/MWire5765-345.jpg

Follow the lead from the horn. By looking at the picture of the relay
on ebay, it appears that it is in the engine compartment, probably
on the firewall or inner fender walls.

Just a quick suggestion.