I've noticed the same on my truck since I installed LED's in all 3 sockets in the cargo/brake light. It's like the circuit carries just enough juice to light up the LED's dimly, but it is not enough to light up the stock incandescent bulbs.
The "factory" installed incandescent bulbs act as a short to ground for the remaing voltage in a circuit, which is normally only a couple of volts. The LED bulbs don't ground out the voltage which is why you get the glowing effect. If you want to eliminate the glow try splicing in a 1.5 ohm .5 watt resistor on the "+" wire going to the led and on every "+" wire to every led bulb. This should help eliminate the glow.
I think it might have something to do with the way it's wired. From what I can remember in my old Ford the switched wire was the ground. So they are constantly fed +12v and when you turn them on they are grounded.