my dad's lexus

Hi all, Joined the forum to fix the odds and ends of my dad's ES300. One (of the many) issues right now is that the door ajar sensor switch on the rear driver's side is broken. More specifically, when I was looking around trying to figure out why the door open warning light was always on, one of the things was I took off the rubber seal to the sensor and something plastic crumbled away. Compared with the other side, the piece that broke off looks like a thin cylindrical cap that, when pushed, compresses the spring, completes the connection and light on dashboard goes away. Anyway, can I replace just that part (It doesn't look like it's got that much wiggle room but maybe I just don't know how) OR do I need to replace the whole sensor? I'm on the verge of just taping the blasted thing down because I'm concerned that it may affect the battery (which I have also had to replace recently but was told by my dad he had it checked a couple of months ago and they said it was fine at the time... ) Sooo anywho, any help would be appreciated. Thanks!