when I need to keep a handgun in the car and I can't keep it on me, I usually put it in a biometric fingerprint safe I have in my trunk every now and then. I just bought a new one a few months ago from ebay, for only 109.99. The listing is under 'Biometric Fingerprint Gun Pistol Safe keyless new NIB' if you are interested.

This may not be the fastest way to retrieve it in a hurry, but its safe and thief-proof.