Electrics Wiring a switched live to a radio which fuse?

Hi Guys

The vehicle is a 1982 T25 2.0 petrol engine.

I have been helping a friend of mine fit a radio to his camper van.

There was an old cassette player in but there was only a permanent live feed to it, no switched live.

The radio we fitted does not have an easy on/off switch.

I looked at all the fuses and looked in the manual and the only suitable fuse seemed to be fuse 12 which supplies the horn and rear reversing lights. This is the fuse i took a switched live from.

With the new stereo fitted and playing we we checking his carbs because there is a petrol smell from somewhere but no obvious signs of a leak (we are suspecting the tank itself), while checking the carbs i noticed they were warm around the choke areas.

I take this is because we have the ignition on to play the stereo.

My questions are, is there a better fuse to use than fuse 12 ie a fuse which is switched live but comes on with the 1st position on the ignition and not when the ignition is turned to the position that sends power down to the engine bay?

2nd, will sending power to the carb auto chokes and probably coil cause any problems / damage?

Cheers