led deterrent and +ves that go to earth?

I was reading something last night on the forum here which has been confusing me for the last 24 hours or so...

I am wanting to install a flashing deterrent LED on a Kawasaki bike... the way i was planning on doing this was running a flashing blue LED in series with another normal CD each drawing about 6 volts giving me my 12volt to wire up to the battery at around about 20mA so the gel cell battery should be happy with the low current draw. I was planning on running a small relay through the 12+ accessory so that the LED is on a normally closed circuit when the bike ignition is switched off and then when the bike starts, accessory line goes 12+, energises the relay and stops the LED flashing. The reason i thought i'd do it that was is that the bike only needs to energise the coil when the bike is running and doesn't need too when the bike is switched off... i figured the current drain of the LED would be a lot less than energising the relay, and less is better when the bike isn't running.

OK... the bit i got stuck on last night was people talking about the accessory wire on a car going -ve when the car is swtiched off and just being able to run a flashing LED directly off that. Why would any +ve wire need to go to ground when the car ignition is off... wouldn't it just be an open circuit? why would it need to go to earth? I'm stuck on that one.

Cheers, Scott

ok that makes good sense, but that makes me think of another forum post that i saw yesterday about a guy who wired up a flashing led and then his amp power light started flashing too... i think i can see what he did.

If i wire the LED up to constant postive and then wire it up to the accessory LED after the ignition (so that it is going to be earthed through the other components on the circuit) it is then going to be flashing my constant +ve through those other components isn't it?! That means they will be getting a +ve through the LED each time it flashes... unless I place the LED after those accessories right?!?!

cheers