I'm posting this information so hopefully it will pop up if someone searches.
I'm installing a trailer brake controller in my '99 suburban. I'm using the A and B posts in the underhood fusebox. Mine did not have nuts attached so... after several trips to the hardware and auto stores the nut size needed is:
M8 1.25
This may be common knowledge, but I didn't know and hopefully I can save somebody a few trips to the store.