Mounted new air horns (12 prostar) can't find wiring to feed em

Mounted 4 buel horns on a board under cab (sounds cheesy but cherry board and it looks good and is solid) ran air from secondary tank to the electric air valve, all good. Just need to track down the location of the original electric air valve that supplies the original air horn. The line goes from horn to top of frame by starter and runs aft. I lose it there. I will keep doing searches for schematics but the specific truck may be a trick to track down. Thanks in advance Mike

 

Thank you all very much, will head out back in the am and try to tie in to the 12v feed to the old air relay. I am increasing the draw (4horns to 1) so I opted for the bigger valve that came with the horns, thus the 12v vs the air line. Oh btw how does one thank...looking for the thanks button I am . Nvr mind found the thanks button...mind is gone good thing I still look good

 

Horns mounted and I used the higher volume air solenoid that buel supplied with the 4 horns. The 4 pack of factory air solenoids on driver's side inside frame near the tail of the trans WAS the correct answer and yielded up the hot wire for the air horn button on steering wheel. After some profanity and a bit of french as well the horns are alive and obnoxiously loud and sweet sounding. Life is good, but for the scraped up knuckles and bruises from squeezing into tight spots. When I pulled the harness going into the four pack of factory air solenoids the ground was first on the left then the factory air horn was next in position 2. Thanks for all your help