66 Cuda: horn works, no other power, no start

Hey guys--I'm getting frustrated with this. Help!

Here's the background: Went to start the car the other day, it started & then quickly died. From that point on, the only thing that functions correctly on power is the horn. No lights, no start. Turning the key is silent, no clicking, nothing.

Have traced some things with the multimeter: I'm getting power to the alternator, the starter relay, pretty much everywhere you'd expect--UNTIL you turn the ignition key. Then the power goes out entirely at those points (but the horn still works).

I replaced the start relay, and it turned over last night--and then it died again. Seems like I've burned out the new relay maybe?

Tested the NSS, seems fine...can get it to turn over by jumping either the starter relay or the starter posts...have tried with 2 different ignition switches, same thing happens with either one...battery seems fine...alternator was replaced last year (although I know that's no indication that it's 100% ruled out)...grounds seem OK...have tried 2 different regulators, same result...

So...is there something that could be burning out the starter relay? Is there something else I'm missing entirely?

any help REALLY appreciated!