I needed to add some electrical gizmos to a vehicle I bought and I wanted to connect these to existing "ignition on" circuits which could handle the additional load (amperage).
Well NOTHING in a car is marked so far as watts or amps the device uses. The wiring has no markings as to size. All you have is say a 20 amp fuse and a bunch of electrical things connected to that circuit.
So no easy way to tell if that circuit is using 17 amps, 18 amps, 19 amps, etc. (and would then have a few amps spare for an additional device to be added to the circuit).
So let's go to the auto parts store and buy a DC amp meter so we can see what these circuits are actually drawing...
Well I went to every single auto parts store and the highest amperage meters they had were 10 amps!
The vast majority of fuses in my vehicles are above 10 amps!
So how is your typical auto repair person expected to troubleshoot and repair electrical problems if the proper tools for the job are not readily available? (I know how to make my own "shunt" using a resistor so that my meter can be used to measure more amperage, but most people would not know this.)
I guess this is why it is common for people to add things to cars and then have trouble with fuses blowing. Then the auto repair person has a difficult time finding/fixing the problem...