I've seen a couple members pull their wall plate to discover it wasn't wired right, or that it was defective, that could be the case here.
Normal operation is to connect park cable outside, turn antenna booster OFF (BTDT) scan channels in cable mode (as opposed to air) and you're good. If it doesn't work this way for you, the first thing I would check is the wall plate, especially since you ran direct and it worked. It could be wired wrong or defective...
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!