The first thing I thought about when I read your letter was the occasional high humidity where you live. I grew up in southern New Jersey and remember when my guitar would be covered in moisture when opening my case. Leaving your guitar case open when you rehearse allows the lining of the case to absorb moisture. On hot days your case can actually sweat and the high moisture content can make your pickup rust and the contacts on the lever switch to oxidize.

Try finding a Desiccant pack which is used for humidity control and protection of materials subject to corrosion or rust caused by high humidity. The container contains granules that absorb moisture. The packet can be put within the instrument case to absorb moisture and can be easily regenerated by placing the unit in an oven for a few hours at low temperature to dry it out. 10 pounds of granules can hold about 12 pounds of water vapor. I would look for a source that sells Desiccant packs. I have often put my open case in a closet with a de-humidifier. I purchased a small one from Sears that works quite well. I was amazed how much moisture came out of the case lining!