Sounds like why I usually let an old truck go.
Yeah, you can keep chasing little glitches but 400k miles of bouncing and jiggling, means the wires will all be work hardened and prone to breakages.
If you don't know what that means, take a paperclip and bend it 180deg. If you bend it back in shape, it will be fine. Do that 10-20 times and eventually it will break due to work hardening. If you only bend it a little bit, it takes many more bendings but with 400k miles, you probably have many millions of little bendings everywhere the wires have the slightest give.
Switches are similar though slightly different. Every time you make a contact, there is a small arc as it engages/disengages. This leaves a little charred spot. Eventually, that starts to mess with the connection (hence, jiggling sometimes works).
If it's sentimental, of course, you can dump money into her and keep her going. Look at some of the rusted out 1930's pieces of junk, that are rebuilt to like-new condition.
But financially, it will probably be far better to replace her.