Ours are made by Autometer but I think yours is from Thunderheart. Contact them as they will be familiar with your setup. Seems they have had as many issues with there speedo as we have with outs.
Here is a link to the Iron Horse speedometer unit which has some electrical info in it. You can call Jason Whitaker an engineer at the Thunderheart factory in Whitehouse Tn and he can answer you questions I am sure seeing how they produce them there.
Neil