Ok, here is a picture of the Aspencade CB Harness
IIRC, that extra piece sticking out near the rubber plug on the bottom left is a square brown 4 pin connector. From what I can see, (but don't quote me on it) pins 13,14, and 15 don't even come into play for the Aspencade on the Aspencade harness. Pins 10,11, and 12 are part of that square brown connector and matches up with a plug in the same area as where the CB gets mounted. It looks like you can probably get a connector here: http://electricalconnection.com/elec...ts/hitachi.htm
I am not positive that this is the exact connector but it will come as a set and you could change out the one on the factory harness if you had to and it should work.
Its possible that 13,14,and 15 may actually connect to the 3 blank pinouts on the AM/FM radio and the wiring diagram just doesn't show it. You may have to use a meter and if there are pins there, test them to see if they match up.
Good luck with this and if you are successful, maybe you can give part #'s that you used and verify the actual wiring. I think I have gone as far as I can. Possibly someone else will come along and add to what I have already told you.
If it were me though, I would just try and hunt down one of the wiring harnesses. I know it will cost you but will be way easier than trying to do this.