cron makes an internal system call to create a process and start the script. The system call is execv - it "loses" environment variables. This is a very common problem for folks trying to use cron.
If you need environment variables, have cron call a script that sets up what it needs when it starts to run. When you login, if you don't run .profile, .bashrc, .cshrc, or /etc/profile you have no environment to speak of. cron jobs do not execute all of these login scripts by default, so they are like a user that doesn't 'login correctly' in that sense.
try starting your script with stuff like this, which is just an example, not a template:
Code:
#!/bin/ksh
. /etc/profile
. /path/to/.profile
# do your stuff here
