By the sounds of things the amp was hard wired rather than using a 12v switch from the headunit so the amp was permanently on.

Basically you should have a good size power cable running directly from the battery to the amp. Where the cable pass's through any metal you MUST use a grommet. You should also have a fuse in the power cable as close to the battery as possible.

The amp should be earthed to a suitable point on the cars body (try to keep the earth cable as short as reasonable possible).

On the back of the headunit there should be a blue wire that is labelled as power cont or remote. This is the wire that will switch the amp on when the headunit is powered up. It will need extending and needs to be run to the rem terminal on your amp.

Just a rough guide mate. HTH