You dove in way too deep before looking at the simple things first. I'm glad you want to troubleshoot your problems rather than taking them to the dealer and we're here to help for as long as you want. Step #1 when there's electrical malfunction , test post to post on battery. 12.6 or higher is full charge, 12 even is only 50% charged and will do ok for testing circuits but you also need to find out why it's not 12.6+ and fix it (from here on this will be called "post voltage). Step #2 after finding 12.6+ . Move leads to cable connectors. ANY LESS THAN POST voltage screams dirty cable connections. Step #3 after finding post voltage at cable connectors, put + probe on + post then - probe on chassis of vehicle. Any less than post volts indicates poor contact between - cable and chassis. As Henro suggested put - probe on - post then proceede to use + probe for testing connections for post volts as they move farther from battery. If and when leads are too short to keep - probe on - post, clean a spot on chassis and connect - probe there. If volts drop 0.1 at any point you have found poor contact/conection,fix it then move on. When you reach a plug , relay or stwith,test it as you have been and I recommend giving it a shot of spray cleaner/lub to prevent future corrosion. Same applies to to cable connections after they are sanded. That covers + connections ,put + probe on + post then test - wires and connections.
Master the above and here's a few tips to speed the process. If a piticular device fails you can usually test switches , relays and connections by turning device on or hooking a load such as tail lamp/head lamp then touching a probe to in and other probe to out. Volt reading eaual or near post volts indicates bad connection,relay or switch. Aligator clamps for meter probes will free up the hand otherwise required for holding a probe. Short pieces of bare copper wire ranging from 16 gauge to 14 guage can be pushed into plugs where meter probe is too large.
I hope you find something you can use in that.