crawl under the van directly beneath the drivers seat look up on the inside of frame rail. There will be several hyd lines going to the unit.
Actually from the Astro 1999 factory Shop manual have the "LT version "
The ABS (Antilock Brake System) valve /pump module with the hydraulic line is on the frame under the driver side.
The ABS electronic control module is under the drivers seat in the bulge under the carpet
The AWD (the All Wheel Drive ) Module (silver case with 2 connectors, one a black and one an orange ) is "in" the drivers side kick panel behind the Parking Brake Assembly" 4 bolts hold the assembly in place after disconnecting the release cable...
Fwiw a common problem is the metal U channel that large amount of wiring runs directly under the carpet here by the parking brake and people not putting wiring and connectors back into channel so the wires chafe on the metal edge of the channel with left foot pressure and cause intermittent shorts
The airbag control module is on the passenger side..