where can I wire my positive cable for my hella horns?

Just use the same wire for the stock horns. That's what I did. I have had no problems.

 

Don't know what hella horns are but if there air horns you will need a relay as they pull alot of power for the compressor. Power the relay off the existing horn lead but then bring a #12 wire out of the fuse box to power the horn. You can bolt it under the front jump point but put a inline fuse on that wire though.

I have one on my G2 they work good. And are really really loud.

 

These are hella horns. MUCH louder than the stock horns.
I just made a short 4 inch adapter cable because the stock wires didn't fit and
I didn't feel to fighting with them. It was easy...

 

Nice diagram.

 

I think that was from there web site. Or maybe as I searched I found it.
But it is nice, and simple. I didn't do the relay though. It's been about a year, and
The wired haven't melted...

I use my horn, a LOT!

 

Get the power from the fuze box.

 

It would depend on if you want the horns to operate when the car is off, or only when it's on.
I think only while the car is on would be best...