original deck uses 5V logic to turn on amp
if you change stereo and use original sub amp - add regulator or something to reduce 12V to 5V
if you use stock stereo and aftermarket amp - you may need to use a relay with 5V control to switch in 12V to amp control/remote
But as the OP says, they want the original amp (so a guaranteed 12-15 year old piece of electronics, that may fail at any time)
I would say put an an alert on eBay for sport trac sub or something.