Courtesy light door switches
On full-size 1964 Buicks, it is very common for the switch boot to disintegrate over time allowing water and dirt into the courtesy light switch. Once this happens, the dome and courtesy light will come on intermittently when the door is opened and is an annoyance. There are a few ways to correct the problem. The two I will cover are rebuilding your switches yourself, or modifying off-the-shelf switches to provide the correct function for less cost than having the switch professionally rebuilt.
Non-functioning factory switch with bad switch boot
Rebuilding your switch
Factory switch with no boot on left, with proper switch boot on right.
To rebuild the switch yourself, remove and discard the broken switch boot. Carefully remove the front retainer by prying the retaining clips away from the housing. Keep a firm hold of it with your thumb because when you get all the retainers loose, you want to let the spring inside push all the internals out under its own force. Not holding on to it means losing small parts. Once the switch is apart, if the delicate copper contact has all four sides to it, you have a rebuilding switch. If not, you will need to fab up your own copper contact block, discard the switch, or search eBay for "Buick switch rebuilding". Clean all the metal items in the switch carefully to remove the oxidation (including the housing terminals), and re-assemble. Lubricate with WD-40 and install a new switch boot.
Modifying off the shelf switches
Scanning eBay, the switch that had the closest external look to a GM switch was this Scott Drake C3AZ-13713-A. Nobody throw a fit, but these are made to fit 65-66 Mustangs. The best part? These are $10 including shipping, off eBay. To get someone to rebuild your factory switches come to around $70, so if you only need driver quality, here is a solution for $10, if you have basic soldering skills.
This is the backside of the Scott Drake switch. As you will notice, it has round female connectors instead of female spade connectors. Also, the switch retaining fingers do not connect the switch to ground, like the Buick switches do.
Why is this a good thing?
Because now the switch can be wired to a good, solid ground, instead of a simple body to switch press-fit.
Here is the procedure: Acquire two female spade connectors and one ring connector from your parts bin, Home Depot, Radio Shack, etc. Solder the ring connector to a 7 inch piece of at least #14 wire for your ground connection. Push the other end of the wire into one of the round connectors on the door switch, and crimp down on the connectors with pliers. Now, solder the wire to the female terminal. Be careful here as too long of a soldering job will melt the switch. Now position two female spade connectors back to back in the other connector. Crimp the connector around them, then carefully solder them to the connector. It may take two passes to alleviate heat to be able to get a great connection for each terminal.
Snap the switch in the factory hole. Even though it is longer than the factory switch, it will not come in contact with anything. Connect the factory dome light and courtesy light spade connector into the spade connectors. Remove the closest vent screw, place the ring terminal behind it after cleaning it, and torque the screw down tight enough to hold the ring terminal tightly to the screw head, as the vent is plastic and does not conduct.
Finished product. The O.D. of the Scott Drake switches are a little less than the Buick switches, but I would bet if a factory switch boot was installed, no one would notice.