I also have one that gives both audio and visual warnings (loud bleeps, series of coloured LEDs). In this case however it's built into a rear view mirror so the LEDs are along the bottom.
In theory then, they should point out to me any "threat" in a way that I'll quickly spot since I should regularly be looking in the rear view.
Does it? Frankly no, it needs to be much more in front and much more obtrusive. This isn't a problem since the thing is so loud you couldn't miss it even with all the windows open - but on the odd occasions I've turned the volume off (usually after driving through town where it gets set off by all the security lights) I've realised it hasn't been alerting me sufficiently.
So, my vote goes with "obtrusively in direct line of sight", but if you can do anything to make it much louder, that'll work even better. If there's an earpiece attachment - can you rig that to an amplified speaker such as one sold for use with a Walkman?
G