WE hear it all day long. Why is it so expensive, it's just my left (or right ) turn signal that is not working. Customers complaining about the cost of an electric repair on their vehicle. Let me explain the difference between a mechanical and an electrical automotive diagnostics.
Mechanical: My car leaks water. We check hoses, radiator, waterpump, and other visible parts that distribute water (or something that looks like water) thru your vehicle. Besides condensation from your AC system (which can generate a nice puddle of water under your vehicle and is completely harmless ) it is fairly easy to detect a leaking radiator , radiator hose or leaking water pump. Diagnostics time under 30 minutes (mostly).
Electrical: my rear passenger side tail light is not blinking when my lights are on, but when my lights are off it operates normally. Now you got a problem! You can't see a leak, you can't smell the problem and your can not hear the error. You have to start at the basics and work your way thru a system of processes and tests. removing lights, panels, test switches, relays, read wiring diagrams
here is one of those wiring diagrams. If and this is a BIG IF. You are lucky you can find the problem within 30 minutes, but more likely then not you will not. Simply because the problem can be anywhere in the car in the circuit between the fusbox and the rear tail light assembly. It can be as simple as a lose connector and as difficult as a broken wire behind a door panel. That is why in electric automotive repair it can take 10 hours to find the problem and 5 minutes to fix it (in the case of a broken wire)