I believe there is a (red) that continues from the horn relay (which connects thru to the terminal block)to the lower quadrant of the fuse block to provide power. I believe it is one of the two top terminals on the lower half. (not at home to look, just going off the 68 wiring diagrams I have saved from firebird.org. I don't think the site has much for the 67 but I assume they are the same in that regard.

http://www.1968firebird.org/68wir/68wir1.html

If you have a voltmeter ground the neg and pull off the lower fuse block connector and check for power there. If you are not getting a reading work back to the relay and so on as shown on the diagram.