Hi - kfk is fundamentally correct.....the reason why a non-Citroen radio head unit will upset the main "computer" (the bit that controls the multi-plex wiring in the car - which is like a USB system on a pc) is that Citroen very thoughtfully twist (or reverse) a pair of data carrying wires within the connectors to the radio. I know this from bitter experiemce (2002 Y reg Xsara with multiplex), when I just tested a Blaupunkt CD radio head for a few seconds. I put the Citrioen original radio back in the car.... when I drove off to work in the morning, I found that I had no "relay" indicator arrows working on the dash panel (and no "sound"). The central-locking confirmation flash from the indicators also had stoped working (although the car still locked). Foolishly looked for fuses, relays etc. Eventually took the car to Citroen, and they did all they could to re-programme the electronics, but the only way to get the indicator "repeaters" to work again (MOT issue and obviously a safety issue whilst driving) was to shell out for a complete new dash instrument panel (which includes some subsidiary electronics). Apparently, because these data wires are "twisted" (and only Citroen do this according to the garage) I had sent a 12v signal up a wire that shouldn't have carried a voltage. Cost about £250 AND the indicators still do not light up on activating remote locking - this would cost another £300! Also the garage informed me that loads of C2's come in for similar work, when owners do DIY up-grades to radios - although on the C2, it's the locking that fails. Thanks Citroen.