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On my car's LW receiver, I have a severe case of alternator whine, especially when the headlights are on. It has become worse over time.
I have read that capacitors might help. However, two things remain unclear to me:
The most important and simplest thing you can do to eliminate alternator whine is run a separate set of power cables directly from the cars battery terminals to the radio. Making sure you include the proper rated fuses links on both leads immediately after the battery terminal connection. (the fuses are for safety and should be on both the positive and negative leads)
Doing that should solve a good portion of your issue.
You should also make sure the antenna is properly grounded to the vehicles body. If it is mounted on a trunk lid you will need to add a ground connection from the body to the lid. As the hinges don't offer a solid ground.
Lastly you can buy a noise filter to put in the power leads just before the connection to the radio. This can be purchase at your every day electronics shack or automotive store. make sure you purchase a filter that can handle the load your radio will put on it.
Alternator whine is caused by a small amount of AC signal component on your DC power supply to the radio. The alternator generates this AC signal as a by-product of how it works. A diode pack on the alternator converts AC current from it's windings into DC to charge the battery. The battery "smooths" out the pulses of energy from the alternator. For more information on this bit, see here: http://auto.howstuffworks.com/alternator3.htm.
To eliminate the alternator whine, you need to remove this AC signal from your DC supply. You mention that the whine has gotten worse, which probably corresponds to the deterioration of your vehicle battery — i.e. it is not smoothing the pulses out quite as well. This is a side issue.
The approach I would use to eliminate the whine noise, and that has worked for me before, is to run wires from the radio directly to the battery, where the smoothing should occur. Adding a filter in the supply cable, specifically the positive cured the issue for me. I used a large DC choke from an old power inverter (used for a similar job inside that) — essentially a large inductor capable of handing the current your transceiver needs — in series with the supply, followed by a large-ish capacitor (25 V, 10 000 µF) on the radio side to dissipate any noise passing through the inductor.
This worked pretty well for me. YMMV.
