I'll keep this simple.

I have a mobile radio mounted in my car. It's an Icom 207H 2m Mobile. I
need to somehow (as easily, cheaply, and quickly reversible as
possible) restrict the transmit distance to something quite short (say,
between 50 and 200 feet, at the maximum). What's the best and easiest
way to do this, while still keeping something that can be easily
reversed?

Will simply running the radio on low power (5W) with no antenna do the
trick, keeping in mind that the radio is inside a metal car? Or should
I somehow shield the end of the coax and/or the body of the radio?

It will only be used to transmit in fairly short bursts, say 10 seconds
max, and at the maximum one burst per hour or two.

Thanks,
A newbie,

J. Antman