In my experience, there are three parts to the job of installing HF mobile in a vehicle: Installing the radio, installing the antenna, and suppressing noise. Of the three, suppressing noise is probably the hardest.
The first question is what kind of noise you are experiencing. If it sounds like a popping noise, and gets louder when you accelerate, it is probably ignition noise. If it is a hash type noise, it is probably an electric motor. The engine fans on my Tercel were very bad, but only ran in traffic, so I just lived with it. I was told that .001uf caps, short leads, from the motor to ground would help, but never did it. Some electric fuel pumps can be very noisy.
One test to see if the noise is coming from the power cable or the antenna is to disconnect the antenna. If you still have noise, work on the power cable. If the noise goes away, the power is OK, focus on suppressing noise at the source. If you connect the radio power cable directly to the battery, you will probably not experience noise on the power cable. (Don't forget to fuse both plus and minus at the battery for safety). Heavy cable is advisable; I would not use anything smaller than 12 ga, and some people use welding cable.
I assume here that you have broad-band noise across various bands. If you have a single frequency birdie, you probably have noise from the computer crystal.
You might check to be sure that you have resistor plugs and wires before starting more serious work. Also, be sure that your noise blanker is on, although most radios do not have super effective noise blankers.
On a 1979 Honda Civic with points, I used resistor plugs and wires, which solved the problem, but meant that I had to change the points every 5000 miles or so, as the ignition system was designed for resistor wires only, and if the ignition system was not perfect, the car ran poorly.
On a 1985 Toyota Tercel, I had tremendous luck suppressing ignition noise by buying heavy copper tubular braid, and slipping it over the spark plug wires. I built a copper box that covered most of the distributor, bonded the braid to the box, and then bonded the box to the block, keeping the wires short. This was a day and night improvement, and I never felt the need to do any more ignition noise suppression.
On a 1996 Subaru Legacy, I again built a copper box, and bought more tubular braid. However, I only got an improvement of about 2 S-units. I needed more, especially on 40 meters, so I ran two ground wires from the exhaust pipe to the chassis. This was probably worth more than 2 S-units, and finished the job to my satisfaction.
If you are systematic, you should be able to identify the source of any problems and correct them.
A couple of other notes on HF mobile installations. Don Johnson, W6AAQ wrote the book, 40+5 YEARS OF HF MOBILEERING. Try to get a copy. I use the Hustler antenna system, and find it adequate except on 75 m. If you plan to operate 75m, consider a WB5TYD Texas Bug Catcher or a Screwdriver antenna. Unless you have an automatic or remote tuning system, the antenna bandwidth on 75m will probably be only about 50kc or so. I have never invested the time and effort required to achieve a satisfactory 75m installation.
November 19, 1996