Answer: I think you need to step back and think about how you want to tackle these problems before you expend a lot of time and money needlessly. You don’t say what year Chevelle you bought, but for the purpose of this discussion it really does not matter.
Electrical problems can be frustrating and difficult to fix. This is because you can’t see electricity. If a mechanical part fails you can simply unbolt it and replace it. With electrical problems you have to be able to diagnose the problem, locate the problem, and finally repair the problem. This makes you a diagnostician, technician, and mechanic all in one. It is for this reason that many repair facilities charge a higher hourly rate to diagnose and repair electrical problems than for mechanical issues.
Since you are on a budget I would focus on repairing the problems rather than replacing the wiring harness. In theory, wiring harnesses are not difficult to replace, but in reality they sometimes are. Seats, door panels, dashboards, carpeting and other components will often need to be removed to access portions of these harnesses. A factory assembly manual will contain the wiring diagrams for your car, but you probably will not need one. Besides, the horn problem and the reverse light problem are not related to the wiring harness.
The reverse lights don’t work because the switch is missing. Switches are available for about $65.00. If the mount for the switch is missing it can be purchased for about $40.00 which includes all of the necessary hardware. Installation is not difficult and the wires that plug in to the switch are probably hanging right there.
The horn problem might be solved by simply replacing the horn relay. If this does not work then something is “grounding” in the steering column. Actually, this is the way that the horn is supposed to work, but it is only supposed to happen when you depress the horn button or horn ring. Addressing this issue will require removal of the steering wheel, which requires a “steering wheel puller.” Once this is done it may be necessary to do further disassembly including removal of the turn signal switch. Unless you know what things are supposed to look like inside of the steering column, you might want to purchase a factory “service manual” which will help aid you in diagnosing this problem. Or, you may want to leave this one to the pros.
Your intermittent brake lights should not be too hard to diagnose. You have already performed one of the most complicated tests used in diagnosing auto electric problems… the “wiggle test,” so you already know that the problem is a loose wire, poor splice, bad connection, or something similar, and you have a good idea of where the problem is located.
Some wiring harnesses such as the engine harness and the taillight harness are easy to replace because they are accessible. This accessibility is what also makes them easy targets for ham-fisted backyard mechanics. Sometimes it is better to replace them if they repeatedly become problematic.