The Low D horn AC Delco D1934 (or) OE 1892162 _and_ High C horn AC D D1939 (or) OE 1892246 both come with a Y-connector pigtail which is a little more than two feet long. Obviously, the length of wire required to go from the OE horns on the left side to the add-on horns on the right side needs to be longer.
The Y-connector should get plugged into the forward most horn on the left side (F-horn) and then the OE harness wire gets plugged into it. In order to make this work, bend down flush the tiny tab on the horn contact, cutoff the loop or a "U" shaped piece on the new harness Y-connector so that you can push the OE male horn harness connector onto the harness.
One alternative: You can either run an additional length of wire (16-gauge dark green) w/ male connector from the female end of that two foot harness wire that came with the new horns or you can splice in a length of fresh wire and run the wire over to the first horn and then using a cube tap, splice into the wire and run a short leg to the second horn, using another female connector.
This is what I prefer to do. Cut off the female connector on the other end of the new Y-connector harness wire, solder an end of fresh 16-gauge wire to it, coat the joint with dielectric grease, slip on a length of shrink tubing, and then run the wire to the new horns (about 4-5 feet). Cut back a 1/2" section of insulation on the new wire about 8" back, fold it and crimp on a female connector. Then strip off 1/4" from the end of the wire and crimp on the second female connector. I would solder all connections and then slip on the plastic housings. Grease the connectors and then slide onto the horns which you previously bolted to the inner fender wall using the holes that are already there. Use wire loom or do whatever you feel is appropriate to finish and clean up the installation.
Parts used from NAPA:
1 ea. #785405 16 Ga primary wire (green) 35'/roll
2 ea. #784491 GM 56 Series Female Terminal Connectors (18-14 Ga)
2 ea. #784531 GM 56 Series Male Housing
Butt connector or shrink tubing
Soldering gun, etc.
and...the Horns: The forward most horn will need to have the bracket reconfigured (shortened) slightly. You can use the F-horn bracket on the other side as a model. However, there is no need to fabricate a position tab on the bracket --- it lines up differently. Use U-nuts nuts, one on each bracket along with the corresponding bolts and fender washers. This will make attaching the horns to the fender a lot easier. A drop of Loctite would be a good idea too. When installing the new horns, I would suggest disconnecting the battery and moving it slightly out of the working area, or BE VERY CAREFUL. Remove the three 10mm nuts that secure the radiator overflow tank to the inner fender wall. Move the tank away from the working area and secure it with a bungee cord to keep it in place.
BTW, after cutting and drilling the horn bracket, paint it to prevent rust. It is galvanized so you might want to use vinegar to neutralize the finish. I didn't!
2 ea. 6mm metric extruded (long) U-nuts
2 ea. 6mm x 16mm (or 20mm) stainless steel bolts
2 ea. 1/4" x 1" stainless steel fender washers