check power in & out of the dimmer on the dash first if there is no power check the fuse even if it looks ok it can be bad , dash lights can be a grounding issue also .
The back up light could be the NSS again trace power to the two black wires going to the trans , you to the trans & back up leading to the reverse lights , if you do have power back from the trans there is a plug behind the driver kick panel , if you have power there check the reverse light sockets , if there is power there then either the bulbs are bad or the ground is bad