Thanks, in the end ( 60 minutes of taking things apart ) I discovered that if you un-bolt the fuse box in the engine bay ( drivers side of this RHD 4th Gen prelude ) you find a gromet in a decent position to get the wire through.

I took the whole of the centre part of the dash out, only tricky piece is that you unscrew the gear stick cover, from underneath.

There is a nice big box down the centre of the car ( for storage in the rear ) that I unbolted and ran most of the cables down the centre of the car, amp mounted in the boot ( trunk ), by cutting a section of foam that pads the boot floor to the shelf that the full size spare wheel is under.

I am pretty sure you would not get two full size Din units in the dash, as the plastic under the head unit is a little thinner.

Finally discovered that my Mk2 was doing wierd things, in that if you switched off the ignition, and then back on Empeg would not power up. You had to yank it out of the dash and pop it back in. The Mk2a is fine though.

Didn't get install pics as was limited by time on the install, as g-f wanted the car back as quick as possible. Could grab some post install pics if anyone is really interested.