I checked the operation of interior lamps on my 2000 GV. When a door is opened, all interior lamps on the ceiling (including the 2 over the driver and front passenger seat) illuminate. Same for all lamps to illuminate if the remote keyless is activated to unlock all doors. If you turn the thumb wheel next to the rotary headlight switch, all ceiling interior lamps EXCEPT the 2 over the driver and front passenger seat illuminate.
So I would agree with others and the thumb wheel dimmer control on the lights may have a problem. I looked at a wiring diagram for a 2000 mini van. There are 2 power sources of 12 volts to the front driver and passenger reading lamps. One is an unstitched, fused 15 amp circuit (#6) from the power distribution center. When you press each front reading lamp on its lens you complete the circuit to ground and the lamp illuminates. That means the ground circuit to the reading lamp is good.
When a door is opened or the remote keyless is activated to unlock the doors, the body control module (BCM) senses this and sends a signal to the interior courtesy light relay to close its circuit and activate interior lamps. It also grounds the circuit to the front reading lamps so they will illuminate. However if you turn the headlamp thumb switch fully to turn on interior lamps, the BCM senses this and activates the interior courtesy light relay but does NOT ground the circuit to the front reading lamps so they will NOT illuminate. So there is a logic path that is being followed to determine which interior device is activating the interior lamps. I would imagine engineers designed the system this way so that the adults could drive at night without an annoying overhead lamp in their eyes but yet have lamps illuminating the rear area so as to keep a watchful eye on the munchkins!
It appears that the circuit through the variable resistor thumb wheel dimmer must be able to pass a minute electrical current in all positions except OF.F That allows the logic circuit within the BCM controlling the interior courtesy light relay to be activated and sensing at all times so it can control the ground path for the front reading lamps.