You should use a relay, powered by your fused antenna lead.  Depending on
your amp's power draw, you may be running too much current through your
switch.  Either way you want the shortest run of proper guage cable as
practical from your battery to relay to amp.

A little tip...don't let the stereo guys talk you into buying monster gauge
cable for your amp.  The IASCA guide book has the gauges actually
required...most people waste money buying copper they don't need.  Make
sure the amp power lead and antenna power lead are fused according to your
amp/relay's draw.  Also ensure your relay is rated accordingly.