The idea of "lock it inside your car" is both ridiculous and frankly, off-topic. It does not even approach answering the OP's question, and offers absolutely nothing useful whatsoever. For all we know, the OP has a car too small to fit a bike inside, or even-- gasp-- a truck.
So to give an actual answer:
I have the "worst" kind of bike rack, which holds the bikes by the top tube. I'd likely prefer a tray-type, but the pillar style rack fits perfectly in the wife's trunk when not in use. It lives in the car year-round.
I use a Kryptonite long-shackle U-Lock and a 15' braided cable. I first put the cable through the two little hoops on the receiver, whose names I do not know. But I assume every receiver has them.
I then weave the cable through both wheels and the frame, then loop back around to the rack's pillar.
I put the ends of the cable onto the Krypto shackle, then put the shackle over the downtube and the pillar, and lock everything together, so that the bottom portion of the lock is facing the bumper.
The 15' cable is easily long enough to wrap two bikes. When I have two bikes on the rack*, I use the U-lock to lock the two frames together, just above the bottom brackets.
Yeah, someone could cut the cable, and then unbolt the entire receiver and rack from the car. But the whole point is to make the bike seem less accessible, and therefore not worth the hassle.
A bike sitting inside a car is protected by a piece of glass. That bike can be stolen with a chunk of brick.
*the rack was sold as a 3-bike, but fitting 3 bikes on it was ridiculous. I cut the tubes down and customized it to fit just two bikes, which is all we're ever transporting anyway.
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