I've done a few of the harness electrical connections, but I've done
all of the bulb connections with
Deoxit D100 contact cleaner, and then a light coating of
DeoxIT Shield protectant.
It's all that AND a bag o' chips. It's kind of expensive, but a teeny tiny bit goes a really long way.
The D Series comes in several different kinds: spray, brush on, pen, little tube like a Super Glue tube (2ml, which is what I recommend if you need it for a few connections and all the bulbs, just couple of drops on a Q-Tip) and different strengths: D100 is 100%, D5 is 5%, etc. You clean the contacts real well with D100, then put a thin layer of the blue DeoxIT Shield on it. I also have the 25ml bottles of both. I will outlive those bottles.
Search the net, particularly places like Amazon and electrical parts shops for the best prices. A 25ml needle bottle is like $24 on Amazon, which is pretty good.
I use DeoxIT for the contacts in my high end LED flashlights, where good connections are critical. About 4 years ago I was tired of the random weird errors with the lights, especially the tail lights, where one would be out and when I touched it to replace it, it would light up. I cleaned all the bulb contact surfaces and the socket surfaces, and used the Shield on them, and haven't had a problem since.
I cannot recommend this stuff too highly.