GMC Repair: Turn signal/backup fuse blowing, turn signal switch, amp fuse

Question
1988 GMC Safari, SLE. Tilt, Cruise, Delay wipers
As soon as the key switch is turned on the fuse pops, no others, just that one. All other fuses have been checked. I have replaced the steering column, turn signal switch and flasher unit with the same results. I pulled the wire harness from the turn signal switch and removed ALL the light bulbs and it still pops when the key is turned to on. I traced the trailer harness back to its connection and removed it out of the system. Same results. An identical steering column came from a junk yard with all the switches on it, no broken pieces. All I had to do was to plug in the wire harnesses.
The best I can figure it's in the backup light circuit/switch. It seams to me that it is the feed from the key switch to the backup lights.  Where is it and how does it work? A wire schematic would help, but I don't have access to one.
Thanks,
MD  

Answer
This is a difficult to fix issue, but I will help you as much as I can.

First what you will need to do is get to the back of the fuse panel. There will be two pink wires coming out from where that fuse plugs in, cut them and put a seperate fuse holder in each wire. Put a 10 amp fuse, or preferably a circuit breaker, in the inline fuse holders, and a 20-25 amp in the trucks fuse holder. When you turn on the key one of the 10 amps should blow, find out what is not working after that, the turn signals or the backup lamps.

After you find out this let me know and I'll get you further along.