[Al Skierkiewicz]

Ikillee,
Same but different, the battery contains a lot of lead plates and connecting bars.
Yash,
Not exactly. The motors can produce interference due to the sparking that is normal in DC motor brushes and commutators. Think spark gap transmitters. The closed motors tend to reduce this spark interference but RF does stray along the motor leads as it leaves the motor case. Twisting the motor leads can help somewhat. Your results will vary.
The effect you experience with a brick wall may be more due to the materials inside the wall than to the wall itself (Pipes, conduit, steel structural materials, wire). If that wall has plaster on the surface for instance, it is likely to have a steel mesh screen (lath) to help the plaster stick to the wall. The convertor makes a lot of RF noise because of the way it switches current to the internal transformer. This produces significant, high slew rate, pulses with lot's of ringing. While the frequency of operation is relatively low, (likely around 100 kHz) it is the ringing that is much higher in frequency and that which causes the interference. when placed close to the bridge, the comparative signal strength from the convertor is orders of magnitude higher than that received from the other radio. This tends to fall off rapidly with distance (following the inverse square law).
I would be remiss if I didn't say something about exposure here. The radio output from wireless home routers is intended to keep exposure limits low and maintain FCC specifications. Please consider what you might be exposing yourself to when making power adjustments to a product of this type, particularly if you spend a great deal of time in one location near the source.