for a start, if you wish to keep everything as it is and change to an aftermarket headunit, you will need a specific ISO connector, that connects up the standard amp unit aswell as the aftermarket headunit.

The power iso is the same as any normal, but the speaker iso connector is different.

The ISO connector needed is aroun £30-£40, any audio shop will know or be able look this up, but you have to be specific by telling them its for an escort cossie.

I have put my lead somewhere as i have reverted to chopping out the standard AMP as it packed up. ( I will post a pic later of it to show you what it looks like)