Backing up a trailer at night with no reverse lights is a real pain!

2005 GMC Yukon

Reverse lights do not come on.

Brake lights - driver's side brake light comes on, but the third brake light and the passenger side brake light do not come on.

It is difficult stepping on the brake when backing a trailer at night to get a little red light behind me from the driver's side brake light.

Here is what DOES work. The tail lights all work. Now, it is the same bulb (3157, I think) on a 2005 GMC Yukon for the brake and tail lights. This means the fuses must be good, too, but I checked them anyway, just in case, even swapping them around with other fuses. The fuses are good. The bulbs are good, but I put new ones in, anyway, just in case.

Well, the third brake light - I do not know if that bulb is good, and I did not try to replace it yet.

So, I took out the passenger side tail light housing and looked at the wire. It goes down to a connector inside the quarter panel sheet metal and connects to some little thin metal box about the size of ten playing cards.

Any ideas or tips for what might have gone wrong?

The interior lights stopped turning on when opening the door at night, too, but I do not know if this is related . . .