An update on my previous question.....
I have successfully re-wired the radio to operate at all times - even during engine crank which is a lot better. I really hated the way the MD head unit reset when I went to start the car...
I have read lots of stories about the voltage fluctuations on the +ive line of the battery during crank, but after running some checks, it doesn't seem to be a problem with the ZR/25.
The only thing you may want to check is the minimum voltage required to keep the radio running. Cranking the engine causes the voltage of the battery to drop by 3-4v. So a minimum operating voltage of 10v is acceptable.
The ISO lead has 2 +ive feed connections, an always on (normally yellow - to keep the radio memory), and a switched feed (normally red) which comes on when the key is in position 2 & 3. You simply patch the "always on" lead into the switched feed pin on the socket at the back of the head unit. You will need to cut the switched feed and bunch it out of the way on the loom.
Be warned that if you are not careful, you can run your battery flat by leaving the radio on. For those of you who have detachable head units - the habit of removing the face will make sure that this does not happen!
Hope this is of interest.