Best Option to Hook Up aftermarket Horn?

[Starwood]

Hey fellas -

Stupid questions v3.0:

The male connector for the 15A fuse on the box is loose as you can see in the pic below. Is it OK the use this loose pin as a power source for an aftermarket horn that is using a toggle switch relay - or avoid it all together?

I ask because when I did try this route, it worked fine for a while, then radio went out, then horn...basically my system DID NOT like it.

Simply put, what would be the best pin option to attached this hot wire to?
Here is a pic of the toggle relay under the dash.

Thanks in advance.