Anyone wired one of these up?

Looks pretty straight forward except for the 'Connect the remaining thin black wire to an ignition activated wire' part. Thumper recommend using the windscreen wiper motor but this is too far away. Is it a simple matter of extending this thin wire or is there an alternate ignition activated wire?

The length of the battery cable precludes setting up the box anywhere but in the battery compartment.

Any help would be great.