Chevy Workhorse parking brake electrical system

Riding down the open road and the parking brake in my Winnebago Brave 33v engaged and stayed locked up. I was forced to remove the cable from the drum brake assembly behind the transmission in order to continue driving. The brake- when working- engages from either putting the transmission in Park or pulling a yellow switch on the dash. Of course at the moment it's engaged under all circumstances.

I've checked the motor to the pneumatics that disengage the brake and the motor works fine, but there is no power getting to the motor. I have read of others who had exactly the same symptoms and fixed it by replacing a fuse that is not part of the factory fusebox but instead they described it as some add-on box somewhere else under the dash. I've been unable to locate any other fusebox. If anyone has any suggestions about where I might want to be looking- I see nothing under the hood or following those parking brake lines from the dash out to under the hood?

Thanks for any insight-