Just because the wire is hanging by the horn doesn't necessarily mean it goes to the horn. They use the same wiring harness on every truck (gas, diesel, Fi, Carbed, etc....) they build, so there will be some unused wires on every truck. That doesn't mean it's not the horn wire, either. So you have to do some exploring.
Is there a wire connected to the horn? If not, examine the horn until you find a connection. If the connectors match, that is likely your wire.
If there is a wire connected, check your fuse. More often than not, the fuse is the problem.
If it's not disconnected at the horn and the fuse is good, the only other point of contact is under the button/s on the steering wheel. Carefully pull it up (they usually snap into place) and check the contacts and connections there. Make sure the contacts are clean and raised enough to make contact when the horn button is pressed. Make sure the wires are connected to the contacts.
If all above is as it should be, your horn is dead. Buy a new one and install it. My personal favorite is the one that plays 'La cucaracha'.
Haynes makes good repair manuals, they're based on a complete tear down and reassemble of the vehicle. They're available at most auto parts stores and cost about $20. Chilton's make some good repair manuals too.
If you want the complete factory service manuals (the one mechanics use in the shop) scroll down to the year of your vehicle on this page
Dodge Ram Repair & Service Manuals
or you can order it directly from the Mfg. They aren't cheap and you won't understand most of what you see, but it is the most comprehensive manual available.
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---------- Post added at 02:24 PM ---------- Previous post was at 02:12 PM ----------
As far as replacing the stereo goes, you don't need or want a manual. Go down to your local Circuit City/Best Buy/Car Audio dealer and buy a harness and trim plate for your make and model of vehicle. Should cost around $40 for both.
The harness will have a schematic on the packaging for your vehicle's audio circuit and the trim plate will allow a standard single DIN stereo (standard replacements size) to neatly install where the factory head unit was removed. Make sure your ground wire is connected securely to a metal surface that connects to the frame or body of the vehicle. DO NOT connect it to the same point as another ground wire or you will get alternator noise coming through you speakers, which is really annoying.
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