2017 Express being fitted with hitch today. Tried to connect brake controller but position #4 in underhood fuse box is not connected. Where can I get power for brake controller?

 

From the "Fuse Block - Body" under the driver's seat. It has a large red wire coming in from the "Fuse Block - Underhood" through a 125A megafuse. Remove driver's seat for easier access.

 

But be careful. that seat has an airbag. It's best to just unbolt it and lean it back out of the way.

 

If you mean the yellow connector, it's probably a seatbelt pretensioner. I've unplugged it dozens of times without issue. I've never heard of a seat mounted airbag, but you guys have fancy new vans so maybe I'm wrong.

 

a 2500 van has got to have a blunt cut wiring harness set up under the dash for the brake controller . put a 30 amp fuse at fuse position 11 in the underhood fuse block and see if your powered

 

The seats have weight sensors that turn on and off the airbags. If you have the yellow wire disconnected and the key on, you may throw and airbag code....
As for the underseat fuse block. I am not sure how much free power is left over from the fuse block. If you overload that 125 amp link, they are not cheap to replace.
Always best to go direct to the battery and add you own fuse since you need 20 - 30 amps or so.

 

That would be the passenger seat only. No fuse block there.