You don't need to find a short. That word is much abused. Shorts cause blown fuses and melted wires. Sometimes, fires. You have no
symptoms of a short circuit.
You have symptoms of a bad ground or bad switch somewhere, or possibly a broken connection or wire. In other words,
an open (or partially open) circuit. That the exact
opposite of a short.
Solving an electrical problem is a step by step process. It's not unusual to have more than one problem, so you need to do one thing at a time to find and solve those problems. Sort of like peeling an onion. You get one layer (problem) taken care of, and now you can see the next one.
It's very common for one electrical problem to mask another, so it is very important to take it one step at a time.
A scattergun approach will leave you very frustrated, and is often very expensive.
So, first things first - clean your grounds. Near each light fixture you'll find a ground wire coming from the light fixture and connected to the frame or sheet metal somewhere. Find that connection (typically a sheet metal screw through an eye connector) and take it apart. Get busy with a wire brush or sandpaper and clean it up. You want to see shiny metal on the connector and where it mounts. Now slap on some dielectric grease (which prevents corrosion in electrical contact points) and put it back together.
Once you have done that for both sides (wouldn't hurt to do front and back, too), see where you stand. If the problem is still there, we'll go on to the next step. Your -20 manual will tell you step by step directions for solving almost every problem, and they are written for 18 year olds who know nothing. They are very helpful.
