Thanks for the information.
Yesterday, I checked nearly every grounding point in the car and also
measured the ohms of every single one of them. At last, the groundings
were not the problem source!
The noise came from the interconnect RCA cables between the radio and the
amplifiers in the trunk. The only reason I can think of is the not-so-good
cable material aged and let the noise jammed in. It was ok in the past
years and it just got noisier recently.
So, cheap things for quick fixes only.
Lewis To, '89 405 Mi16, Hong Kong.