Welcome to the forum and irv2.
Here is a
Brochure and
Owners Guide for you and below in my signature you will find links that will relate to your Essex also.
The lock switch has picked up moisture and the internals have rusted or corroded so switch does not function properly.
In
QT's 1 & 2 you will find slide information and some switch threads that others have posted that may help you, here are a couple of them
#1 slide,----
#slide switch.
They now have a slide lock rubber seal to help keep moisture out when slide is extended.
Other
slide seals.
More
slide information with links in post.