Grounding or Bonding
Bonding, is one of the most important aspects of mobile radio.  There are several reasons for this. First is noise abatement. Bonding minimizes the leakage of RFI into and out of the various bolted on parts of the vehicle. It is not uncommon to see a 20 to 30 dB drop in received noise levels once they're properly grounded. Bonding engines, hoods, and in some cases radiators is important for the same reason.

The best mobile antenna money can buy, isn't any better than the ground plane it is mounted over. Maximizing the available ground plane is what bonding is all about. On the HF bands, our vehicles act more like a capacitance to ground, rather than a ground plane. The bonding doors and trunk lids has a lesser effect on noise, but does aid in maximizing the ground plane.

If ground losses are high, it doesn't make much difference how good the antenna is, because ground losses will be the largest factor in determining efficiency. If ground losses are low, the difference in efficiency between a cheap antenna, and a good one become very apparent.

Ground Straps
One of the most misunderstood concepts is the difference between DC and RF ground (neither one is a ground plane). A ground strap may work perfectly as a DC ground, but at some frequency that same ground strap will make a perfect antenna! We all know that an inductor can provide a good DC ground, but look like an open circuit to RF. And that a capacitor can provide a good RF path to ground, but not a DC path. Ground strap, like any piece of wire, has both inductive and capacitive reactance. These reactance's change as the frequency changes. For any given value of reactance, as the frequency goes up, inductive reactance also goes up, but capacitive reactance goes down. When inductive reactance and capacitive reactance in any given piece of wire are equal, that wire will become an antenna, and ceases to be an RF ground. There are a few things we can do to assure both a good RF and DC ground.

One of these is to use flat braided wire. RF flows at the surface rather than through the wire, and flat braid has more surface area for any given current carrying capacity.  It has more capacitive reactance which increases the self resonant point. Flat braid is also much more flexible and less likely to fail due to repeated flexing.

For longer lengths, one inch wide braid is a better choice. But keep the length short possible. Remember, the ground strap must present a low impedance connection to effectively shunt RF to ground.

Good connections are also important to provide both a DC and RF ground path. Crimping and soldering are mandatory. Crimping provides a good mechanical connection, and soldering a good electrical one. Good quality lugs and connectors are a must too, as the cheap ones do not solder well.

How To Make & Attach Them

There are few things to be aware of before you start drilling, and attaching ground straps.

Vehicles have dozens of wire looms placed throughout the superstructure. This includes the A, B, and C pillars, under both door sills, inside the doors, under the carpet, and behind almost every piece of trim. If in doubt, find out before you drill or screw.

When bonding the tail pipe for example, there are usually strengthening members, or existing mounting bolts which can be utilized. The bottom edge seam of the rocker panels is also a safe bet. Some vehicles have predrilled and tapped holes for accessories and/or for assembly purposes. They're usually metric which will necessitate a trip to the hardware store. What's more, they're safer than drilling into panels unless you know for certain, nothing is behind them.

While you're at the hardware store, pick up a supply of star (serrated) lock washers. The preferred type have both internal and external teeth, but are not always available in the smaller sizes.

The actual screw type is up to you.  Number 10 self-tapping sheet metal screws work well if the material is 12 gauge or less. Self-drilling ones work better for thicker material.

Remember to crimp and solder them as crimping alone allows moisture to seep into the connection with predictable results. If the strap is subject to abrasion, cover it with heat shrink tubing.

Where to Put Them
Caution is also needed when bonding the doors, hood and trunk. You don't want to end up drilling through the lid, or into a wiring harness underneath! Keep straps away from away from hinges, door stops, and weather seals. Hoods and trunks should be strapped across both hinges.
Almost without exception nowadays, exhaust systems are made of a good grade of stainless steel. Wire brushing a small area for the lug to bite into and using stainless steel hose clamps to secure the lug works.  The opposite end should be attached to the under frame or unibody strut work. If the car is undercoated you may have to clean a small area.

If your vehicle is body on frame you'll need a bunch of straps to go between the body and frame. As an example, four separate straps (one on each corner) work well for a pickup bed. Incidentally, don't rely on any factory strap to provide a good RF ground. They're meant solely for DC grounding tail lights etc. and are just inadequate for RF grounding needs. The same can be said for factory engine strapping.

Depending on the vehicle, there can be several dozen other places where ground straps will provide a benefit. These include, but are not limited to, bumpers, bumper backing plates, suspension parts, rear axles, tailgates, or virtually any bolted-on piece of hardware.

Engines are also over looked because most of them have visible ground straps. They're for DC of course, so adding wider ones can help curb ignition noise. 

If you'll just take the time to do your bonding correctly, you'll be amply rewarded. Short cut it and the results won't be worth the effort.

There is one more aspect of bonding worth mentioning. Don't confuse DC and RF grounds. Some amateurs believe a good DC ground is a substitute for a ground plane. Think about this for a moment. A 1/4 wave vertical antenna (loaded or not) is one half of a dipole. When used as a base station antenna, a vertical must have a number of radials under it. These radials act as the missing half of the dipole.

In the case of a mobile antenna, the missing half is supplied by the vehicle, and its capacitance to the ground underneath the vehicle. Since ground loss is the single biggest factor with respect to efficiency for any vertical (especially a mobile one), maximizing the ground plane is essential. As with the number of radials, the more you have, the better.

At the very least make sure that your mount has a good bond to where ever you install it. What ever the antenna is installed on (hood, trunk, hatchback, pickup bed) is going to be your ground plane. Not having a the mound bonded is the the number one cause for an antenna not to tune properly.