The intermittent short is very hard to find because you do not know when the short is going to come. When wire is shorted permanently, I use a device called Short Finder. It works well for me.
The device has two parts. One part is a flasher type box which is to replace the burn-out fuse and the other part is a very sensitive magnetic field strength meter. The short causes the flasher box on and off repeatedly and you use the meter to trace the strongest magnetic path. You dig in the place where the strongest magnetic path drops sharply and there is the short. The meter is very sensitive. You can detect the magetic field along all wires (thru walls) sharing the same fuse but the strongest path is the one with the short.
I got the device 20 years ago from J.C. Whitney & Co. (www.jcwhitney.com) for about $20. J.C. Whitney is still selling it for about the same price. Harbor Freight Tools (www.harborfreight.com) has it too.
I use it on older cars. I am not sure if it is good for new cars with all the electronics (I am worrying about the flashing causing power surge).