You obviously know the antenna is rather lossy to begin with, so anything you do isn't really going to improve its efficiency because the Q is less than 25, best case. But, you can certainly make it worse.
I don't remember the URL, but some amateur in Australia, placed caps between the taps essentially turning the antenna into a trapped, multi-band vertical. I suspect what little efficiency it did have, is now even worse. The only advantage is instant QSY without getting out of the car to change the taps.