Silverado Electrical Question

2006 Silverado 1500 Crew Cab. Trying to wire in a wifi transmitter for a rearview camera. I don't want backup only, so I'm not wiring it into the backup light circuit. Here's the issue: if I connect the transmitter leads directly to the battery, it powers up just fine. If I connect it to the cigarette lighter or the DRL circuit (these being switched circuits), I get nothing. The meter says there's 12VDC wherever I connect it, but the transmitter isn't getting any of it. It only draws 300 mA, which shouldn't be a problem on a 10A or 15 A circuit. Fuses are all good. Any ideas? Thanks!