I have a 1991 Subaru XT6 that has the various interior lighting, such as the dash illumination as well as the radio light backdrop that seems to pulsate between various stages of intensity. This does not change based on engine rpm.
THe battery and alternator or in good condition, at least as far as a plug in diagnostic tool that fits in the lighter and displays voltage and has three LEDs.
Can't quite figure the cause.