Why would passenger side power window go up but not down on my 1990 astro van?

A faulty power window switch or faulty wiring. The motor works in either direction (there is only one motor per window), so it's not the motor but the wiring or the window switch. Unless the motor control is damaged, that is. Check the switch for continuity first, then check the continuity of the wiring. If you don't find the fault in either of those places, check the motor assembly. Post back here with your findings. Thank you.