There is a difference between Electrical Ground and RF Ground. The VOM reads Electrical Ground. You need more expensive test gear to read RF. The best and cheapest this is to be sure that you have good,Clean (wire brushed or dremmeled), tight physical ground connection between the antenna and the ground surface. Something I forgot in my other post is that you need a good physical ground between the Radio chassis and the vehicle chassis. The can be acheived if you have a large screw on the back of the radio or drill a hole in a clear area of the back plate and, in either case attach a flat copper braid or flat copper strap with stainless steal nut and bolt or replace the chassis screw with stainless screw and stainless star washers. Seal the connections with RTV silicone or Plasti Dip. This seals out moisture and prevents the hardware from working loose. If you don't seal them, check them for tightness monthly. A ground that is not very tight can cause static interference. It may look tight, but tighten it to be sure. The copper strap is the best grounding material because RF travels on the surface of the copper not inside and strap has more surface area. I mentioned tailpipes before. The are a leading cause for noise in ham HF (CB frequencies fall in that range) frequencies. When grounding tailpipes it is best to use a stainless steel pipe strape to secure the ground and use a high temp sealing material.
One other point is to solder all connections, no clippies.
If you want lots more info on mounting and wiring and grounding check this site:
http://www.k0bg.com/ Alan is what is called in HAM an ELMER. That is a very experienced HAM who helps newer HAMs figure it all out. HAMs by nature are experimenters and are always tweeking and seaking better ways to get the most out of the least. I have spent a great deal of time asking ELMERs for advise and Alan is one of the most knowlegeble and acurate ELMERs I have found.
Al this may be a bit of over kill for raw CB, but I have had CBs in many diffent vehicles and building and have found that with a few simple steps you can get a lot more from your equipment.