How to find a good ground

[ohio_caps]

Well being the car novice I am slowly getting my auxgmv3 installed. Got the radio pulled, hooked up the auxgmv3 and got the dash put back together. I am now looking for a good ground my the glovebox or radio. Is there a way to use a multimeter to find a good ground close to the radio? Anyone know of a ground close to that point? I was thinking of checking resistance from a bolt at the door hinge to metal object behind the glove box.

2004 Trailblazer LT

P.S. I am still looking for the best switched fuse in the rear fuse box to use too.

Thanks ahead of time.

[ohio_caps]

Wverything is done and running. Thanks to those that sent messages.

[jd-autotech]

there are two grounds in your aldl connecter under left side of dash top row middle next to each other and the bottom right of connecter is pos. chec it out with your meter or a test light