ok so have just noticed some thing odd i get 12v on the switched line along with the constant line all the time as a result when i put a new head unit in the lights will stay on when i take the keys out the ignition???? so i don't think it will matter whether i swap the lines over?
there is usually a difference in the thickness of the wires, which dictates how much current can flow and therefore how much power can be supplied to the headunit. therefore, you should always use the dedicated 'constant'/permanent/BATT +12V wire (they can have different names) to connect to the YELLOW constant wire on the new headunit. the switched/ACC/IGN wire is often thinner, serving only the purpose of triggering the headunit on/off via the ignition key.
in the alfa, i can't remember if there was a definite difference in thickness. but another factor is the fusing; the constant wire may have a bigger fuse or be used to supply only the headunit. the switched wire (or equivalent) may have a smaller fuse and supply multiple other items.
so you should use wire #7 as the constant supply wire in the alfa ISO plug, and connect to the YELLOW constant wire on the new headunit. #4 may be a switched wire if it exists (varies from alfa's to alfa's). this is of course swapped around compared with conventional ISO wiring patterns.
for details on the GT wiring patterns, have a look here. will be near-identical to the 156 (see TallPaul's link to the .pdf in first post, esp about Plug A).
and will have to get a head unit with a removable front??
that is one good way to do it. when you remove the face, the unit switches off automatically, and you will soon be in that habit. if you leave it in place at home overnight, manually turn it off (via button).
a slightly more complex wiring method is to wire up a little switch (toggle/rocker) along the wire supplying the RED accessories wire on the headunit. extend the wiring and mount the switch somewhere like under the centre console where you can reach it but is somewhat hidden. you can then manually switch it, and it will operate like a 'switched ACCessories' wire.