You can get a 2001 Chevy Express wiring diagram at your local library. Most libraries will have the Chevy service manual with the wiring diagram.
A likely cause is a faulty interior light dimming switch. This switch is directly connected to both the dash lights (for dimming), and the dome light (on/off).
check the wiring. its probably shot
Wiring lights on a Chevy truck is simple. All you have to do is color code the wiring and tape them together.
either your fuse is blown or the wiring is bad
Take it to the garage.
You can find a wiring diagram for your 2004 3500 Chevy Express tail light amp most Chevrolet dealerships. Chevrolet service manuals will have the diagram and can be obtained from most local libraries.
A person can get a wiring diagram for this car as a print out at an auto store. They can also find the wiring diagram in the maintenance manual of this vehicle.
go to Autozone online. They have repair info as well as wiring schematics
Blown fuse, both lights are burnt out, There is a wiring or ground issue.
Probably a loose connection in the wiring or the bulb socket.
Check the bulbs first, then the dimmer switch and wiring.