Looking at the schematic for the Winegard, the cable in jack is disconnected when you turn the amp on. The switch is double throw, double pole, with one side used to apply power to the amp and the other used to switch inputs.
You might get a little "leakage" signal to the cable jack, but not enough, I'd imagine, for a decent signal.
2005 Georgetown 342DS XL, Full Banks Package
2005 Liberty Renegade 4X Toad
1989 Wrangler Sahara Toad
1984 19.5' Beachcraft i/o 5.o liter, 4-bbl (and you thought your Moho sucked gas!)
D/W Carole and Maggie the Teacup Lab (Black Chihuahua)