Can't get uninstall custom action to run

[Azmaeus]

I created a simple custom action that runs 'echo "Test" > c:\test.txt', In-Script Execution I've tried both immediate and deferred execution, Install Exec Sequence I've tried "After InstallFinalize" as well as "After DeleteServices" and a couple others, and Install Exec Condition is set to 'REMOVE="ALL"'. When I try to run this, I get the message "Error 1721. There is a problem with this Windows Installer package. A program required for this install to complete could not be run. Contact your support personnel or package vendor. Action: MyCustomActionName, location: C:\...\test.txt"

In the InstallShield log I see:

MSI (s) (24:20) [03:39:34:735]: Doing action: UninstallRedistributable
Action 3:39:34: UninstallRedistributable.
Action start 3:39:34: UninstallRedistributable.
UninstallRedistributable:
Action ended 3:39:34: UninstallRedistributable. Return value 1.

I've also tried running other software and I get non-zero return codes. One was a simple program I created that logs what parameters were passed to it, and it's not logging anything.

The ultimate goal is to get a SQL Express install to uninstall a specific instance.