Headlights 2000 Chevy astro van dont work they come on some times but go right off i think theirs a ground loose or some thing new battery new alternator

I'd check the headlamp relay in the under hood fuse / relay box to start . The relay is energized by the BCM - body control module !
Voltage is available to the headlamp and panel dimmer switch at all times. When the headlamp and panel dimmer switch is activated, the two following actions occur via CKT 352 (WHT):
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A headlamp enable request is sent to the body control module (BCM)

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The headlamp relay is activated

The VE1 option is equipped with automatic headlamps, which is activated when the photo sensor detects dark conditions. A code will set if there is a fault in the photo sensor, or in the two CKT's between the photo sensor and the body control module (BCM).
The coil output of the headlamp relay travels through CKT 250 (BLK) and is grounded at G101.
When the headlamp relay is activated, power is supplied to both the LH and RH headlamp fuses through CKT 1346 (LT BLU).
LO and HI beam functions are activated/determined by the multi-function switch on the steering column.
The HI beam function must be enabled by the body control module (BCM).
The headlamp power travels through the multi-function switch at the steering column and is grounded through CKT 750 (BLK).
The flash-to-pass function is also activated by the multi-function switch on the steering column. It is powered by CKT 1840 (ORN) and enabled through the body control module (BCM) via CKT 307 (YEL).