In a word, grounding. The first place to look is the antenna mount, where it
attaches to the body. (I don't know where your '99 antenna is located, but
if it is a fender mount you will find this part at least easy enough.) If
you can unmount it, clean up the contact surface as much as possible. Often
there is paint inside and out - great for rust resistance but awful for AM
reception - that will have to be cleaned away with sandpaper or Scotchbrite.
Applying silicone grease to the bare metal will prevent rust while allowing
the antenna to make contact.

Back when I was in avionics we ended up spending most of two hours
installing ADF antennas. ADFs are basically AM radios with fancy antennas
and some circuitry to indicate which way the station is. If we just slapped
the antenna on the skin, like we tried at first, the interference was
terrible. We sanded down to metal and got decent reception. Then we found
that treating the aluminum skin with Alumiprep (an acid) and Alodyne (some
other sort of chemical made for the purpose) that the performance was much
better. Fortunately, steel isn't that touchy.

Mike

it could be the spark plug wires also. good well insulated ones do cost alot
but could cut down on the humming u hear. also maybe just buy a better radio
if u can afford it.

                                -jeff