I just replaced the slide assy to no avail. when slid open the backlight fails to go on and the phone also will not unlock when slid open as it used to and should. This has nothing to do with the ALS. The keyboard is still functional (in light). Please let me know what the problem might be as I have replacement parts for almost everything on hand.

Well I've figured out that it must be the magnetic reed switch... I guess I'll change my question to: How do I fix that?

Had exactly the same problem, answer is here.

I am aware this thread is quite old now but thought it will be better to leave it answered.