Mk3 gti 16v: original stereo not working

[mcashen]

i took the original sony xr6759 (vw branded) cassette player out of the car and ran it with another sony cd unit for a few months.
I've put the original stereo back, and but i get no display. I know there is power reaching it, as you can hear something click when the reset button is depressed. My electrician mate has tested the power feed is live also.
Any ideas? Is a code needed ? I'm the second owner of the car and do not have the manual for the stereo.

[Onge]

Double check your constant live fuse in the car. I had similar problem last week, where the unit would only eject tapes (electrical eject), no display or anything else. The constant live fuse had blown.  

[Overseer]

^  also related to that check that that its not been ISO'd as by default the constant and switched live feeds are swapped over.. the sony is VW not ISO.