I had a similar problem with my Class J (also bought at an exhibition) and obtained a new harness from Bachmann. The original one seemed to be made with bare stranded wire covered in a sort of matt black paint - which had crumbled. IIRC, the replacement was with 'regular' insulated wires.
You may be having a similar issue.

Regards

I can't see it being practical for them - although it might save you a little bit of money. I'm not sure what Bachmann might get out of it anyway; I certainly don't think it will increase sales of new models.
You can always sell your 'obsolete' models on one of the many private trade or auction sites. Many people either aren't able or willing to keep buying the latest issues, which may not suit their layout anyway (why buy a new digital, sound equipped loco to go with an older analogue layout?) but will happily add to their stock of less modern items.

As ever, your mileage may vary...

I have three Bachmann locos which are factory equipped with decoders - an Alco RS3, Alco 2-6-0, and a GE 44-tonner. The RS3 is no trouble (albeit slow compared with a similar Atlas greyhound!) in that I can reprogram any CV quite happily. The 44-tonner and the 2-6-0 exhibit a peculiar error in that I cannot set CV29 to any number which uses the 28-step speed table (CV67-94). Both decoders will accept the speed table, but won't acknowledge the CV29 values to use it!
Programming using JMRI gets an error message that it receives no Confirmation from CV29; using my MERG CANCMD and CANCAB shows CV29 to program OK, but re-reading, the value shows no change. It seems, therefore, to be a decoder issue.
The literature assures me that the decoder (the exact model is not stated) in both locos can use 28-step speed control, so I am a little puzzled as to why they will not accept 28-step programming.

Any ideas?

Barry