Where is park brake pressure sensor for 2015 peterbilt 579?

The brake light on your dash is triggered by the absenceof air pressure to the spring brake chambers, not the presence of it... that circuit starts in the air drier, but that's likely not your problem... PACCAR has recently released a series of required updates both for their MX engines, and for the cab CECU (the latter applying whether you have the MX or the Cummins ISX), and the cab CECU needs to be reflashed with the body control module program - ESA. Don't think you'll get a hold of ESA and do it yourself... if you don't know what you're doing with that program, it's extremely easy to end up blank boxing the cab CECU.