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A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a 12 volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it's not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.

For intermittent problems like yours the bulb may be dim already. Watch what takes place when it gets bright. That's when the short is occurring. It could be due to the rocking of the engine when you shift between reverse and drive. It could be due to the body flexing when you drive over bumps in the road. The bulb limits current to a safe value when the short occurs, in this case about one amp. If the engine won't start with a brake light in the circuit, try a headlight bulb. The low beam filament will limit current to five amps, and the high beam filament will allow about six amps to flow.

This question was posted twice, here is another response.

CJ MEDEVAC
Nov 3, 2015.

SOUNDS LIKE THE ENGINE IS TORQUING IN ONE DIRECTION WHEN IN FORWARD GEARS.

IT TORQUES THE OTHER DIRECTION IN REVERSE.

YOU SHOULD BE ABLE TO SEE WHICH IS WHICH IF YOU REPLACE THE FUSE, WITH THE HOOD UP, AND A PAL EASING OFF OF THE CLUTCH IN REVERSE

(I'M JUST SETTING YOU UP THUS FAR)

NOW THAT WE KNOW WHICH WAY THE ENGINE WILL TORQUE IN "FORWARD"

WE MUST SORTA GUESS THAT A WIRE IS SHORTING WHENEVER THIS HAPPENS

SORTA GET IN THERE AND SEE WHAT WIRES MIGHT MOVE AROUND DURING THE TORQUE. ALSO SORTA LOOK TO SEE WHICH ONES MIGHT RUB OR GET PINCHED.

(YOUR ENGINE MOUNTS OR TRANNY MOUNTS COULD BE WORN AND LET THE ENGINE TORQUE MORE THAN IT SHOULD, THIS MIGHT ALSO CONTRIBUTE TO THE SITUATION)

ANOTHER POSSIBLE "TEST" IS TO HAVE YOUR PAL TRY FORWARD GEARS, WITH THE WHEELS CHOCKED REALLY GOOD. THIS WILL "LOAD" THE DRIVE TRAIN AND MAXIMIZE HOW MUCH THE ENGINE TORQUES. THE KEY HERE IS TO BE WATCHING THE WIRING AND WHERE IT IS WHEN THE ENGINE STARTS TORQUING. HAVE YOUR PAL LET THE CLUTCH OUT IN "MEGA SUPER SLOW MOTION" SO YOU WILL HAVE SOME OBSERVATION TIME!

NOW THAT MAYBE YOU AND I ARE ON THE SAME PAGE

LET'S USE A "CARADIODOC TECHNIQUE" ! (HE'S ANOTHER EXPERT ON HERE, A RETIRED SCHOOL AUTO MECHANICS TEACHER)

HIS TECHNIQUE WILL WORK WELL, IT HAS FOR ME, AND MAKES GOOD SENSE!

I WANT YOU TO READ THRU HIS ANSWERS IN THIS LINK

http://www.2carpros.com/questions/brake-light-fuse-keeps-blowing-but-found-something

THE ONLY THING I MIGHT ADD TO THIS IS, YOUR VEHICLE MAY HAVE TO BE RUNNING/ THE KEY ON AT MINIMUM. I'D USE LONG WIRES TO GET THE BULB WHERE YOU CAN SEE IT WELL, WHILE YOU ARE PULLING/ SHAKING/ FONDLING THE WIRES IN QUESTION .THUS THE REASON I WANTED YOU TO SEE "HOW" AND "WHERE" THE WIRES MOVE DURING THE TORQUE. THIS MIGHT EXPEDITE FINDING IT.

PLEASE RETURN WITH GOOD NEWS, OR ANY OTHER QUESTION YOU MAY HAVE

THE MEDIC