Winegard TV signal booster not working

There are 3 coaxial cables coming into the back of the unit plus a wire which I assume is the hot wire for the power to the unit. I can find no fuse! There is very little room to work and the cables will not pull out from the wall more than 2" to 3". I did check for 12VDC and there is power coming into the unit. I have no idea why there are 3 coaxial cables (seems like there should only be 2 (one in from the antenna and one out to the rear TV) - - and I don't know which cable goes to the antenna.

I assumed that when I pressed the "Off/On" button and the light came on that I had 12VDC. Is that not correct?

The booster is actually in the head. The unit on the wall or ceiling is the power supply that induces 12 volts DC onto the coaxial cable.
First check the fuse on the back of the unit. Pull it out of the wall or ceiling. check to make sure 12 volts is getting to the unit, check to insure 12 volts is on both sides of the fuse.
Probe into the coaxial cable center pin to insure 12 volts is going toward the antenna.
Reconnect everything if 12 volts is going toward the head. If 12 volts is going toward the head, go onto the roof disconnect the coaxial cable from head. Check to see if 12 volts is present on the cable. 12 volts present would mean head has failed and needs to be replaced. No voltage means cable has an issue and needs to be looked into.

They can. Have you checked it for 12VDC?