Make sure your receiver antenna isn't touching/resting, or strapped to anything electrically conductive... Graphite/Carbon Fiber is conductive. Insulate your antennas well if you must have it touch something like that, aluminum, steel, these are all conductive...

Also, make sure there's enough antenna exposure. Bundling up your antenna and sticking a bit out will reduce your range to some effect...

Also, make sure you turn on your transmitter first, before turning on your receiver...

Lastly, this is a long shot since you claim that it works great in other cars, make sure you're not being jammed. The AM scheme is more prone to jamming.

I really don't think it's a motor issue. Motors usually will draw current if speed control is putting out a signal (switched power)...