Find the remote lead that goes to the aerial, cut it and fit a small push switch somewhere under the dash or in the centre console.

Its

*usually* blue, on an aftermarket stereo.

Bear in mind if you have an amp, that it will normally use the same wire to use it to remotely switch on

(so cut after the amp joins it!)

My high end Pioneer av system does this.

.

.

whereas the previous pioneer headunit

(a jap one), only extended the aerial once you selected it.
