This comes at the perfect time as I am completely rewiring a 69 ply wagon for a client. We started off with a generic 21 circuit harness off ebay and I am in the process of installing it now. This isn't my first rodeo as I have done a lot of electrical work on past cars, most recently built a harness for my 73 Cuda restomod.
First off, while the harness will provide you with all of the feed lines and connecting lines generally, you are going to have to understand how everything interrelates and connects, what does what and make some decisions on how you want it all to function. I am not familiar with the harness you bought so I don't know if all of the switch connections are already in place or not, but either way, the very first thing I do is make drawings of every switch and subsystem I am going to be using in the car. I do mine in PowerPoint because it is easy for me but you can hand draw it or even use a more complex program. I put all of this into my "electrical" book for the car along with copies of the factory diagram/s for reference. To draw the switches, I use the factory diagram to see where everything goes and/or connects to which then helps me understand how that switch was designed to function.
Ma Mopar did things a bit convoluted in my opinion electrically, but it is important to understand why and how it was done, this will help you apply your new harness so that everything works.
While I could probably list a ton of what I would call tips, here are just a few;
1. do no assume everything in the car works (meaning switches, lights, etc.), you have to test everything.
2. as already stated, terminate your wires correctly which means proper crimp, solder, heat shrink. EVERY connection I do is done this way.
3. buy good tools and supplies, this doesn't mean the most expensive it means quality crimpers, wire strippers, etc. as well as spade terminals, Packard 56 blades, etc. You can get everything online (much of it on Amazon) but look at Del City, Waytek, etc. Get a good soldering system, I use a Weller digital but have used many others, it needs to get hot enough to heat up the joint and melt the solder.
4. if you are not familiar with soldering, learn it before starting. It is not hard but like anything, there are some nuances such as tinning the solder tip, using flux, selecting a type of solder, etc.
5. buy 3:1 heat shrink, this means it will shrink 3x its beginning size which gives you a lot of working room. Not a lot worse than crimping and soldering an end and then when heat shrinking finding out it doesnt go down far enough to protect the joint.
6. take your time. If something isn't right, fix it correctly before moving on.
7. do not take shortcuts (like twisting 2 wires together and tapping them up), this will bite you hard later
8. do no look at the entire project holistically, focus on one area and work that, then move on
9. lay out your harness at the beginning. I am not sure if you have the dash out but I HIGHLY recommend it as it makes it so much easier to deal with.
10. understand how electricity and specifically automobile electrical systems work along with how relays function. Most importantly understand the fundamentals which sounds intuitive and simple but can cause problems such as with negatively switched items such as the dome light and some fuel pumps.
11. make sure you install good grounds in the system which includes the instrument cluster, dash frame, engine, etc.
12. do not enclose your wires until you know everything works. I use electrical tape (3M Super 33) and put a small wrap every 3-4" to hold it all together first. Yes, this means that you will have to do some disassembly later one to wrap if you desire, but its a lot better than having to cut the wrap off to figure something out.
13. keep good records of what you do, this might prove very important later on if there is a problem. Get a notebook and make drawings, write notes, etc. so you have something to refer back to. Include any instructions or diagrams you get along the way for the harness and/or other things such as a MSD, new gauges, etc. My electrical books (3 ring binder) is typically several inches thick when I am done, but it has everything you would ever want to know about the electrical system of that car.
14. as stated, do not cut your wires until you have routed them and are happy with the length.
15. do not apply power until everything is connected or you are absolutely positive there are no bare wires laying around or hiding.
Have fun
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