Actually, I don't use a SWR/wattmeter in my mobile as I let the auto-coupler worry about matching the antenna. But I do have a suggestion or two in case you need to use one.
Most of the small ones meant for mobile use are very inaccurate; in the order of ±25% perhaps more. The larger ones tend to be more accurate, but are harder to mount.
The best ones are those with remote sensors. This typically makes the meter end smaller, and the actual bridge can be mounted near the antenna feed point.
One last point. The majority of the units built in to most rigs are also grossly inaccurate.