[VetteVet]

After looking at the website, I can't determine if the control module uses the positive or negative circuit, or a combination of the two. You should be able to answer that question as you wired it into the wiper system.
The stock system is wired as a constant positive with the switching done on the negative(ground) side. Removing the fuse kills the power to the motor so I would assume that the or the wiper switch could be where the problem lies.
There could be a short in the switch between off and the low-speed wire on the switch or there could be a break down in the module which only allows the low-speed function to work.
I would begin by removing the switch and checking it for correct functionality with a multi-meter. If that checks out ok then I would remove the module and wire the system back to stock. If everything works as expected, then I would contact the vender and ask them if they would exchange the module for a new one. They may or may not do that depending on the amount of time that you have had it installed.