I have a (02)1.2 Corsa SXi with very similar problems, including erratic non start. Your answers were very helpful.
I had problems getting into the dash, below and to right of steeing (2 screws, HT type), but once I removed them a bit of brute force was required to pull the plastic cover upwards and away from dash. Once inside I could see a horizontal blue/black plug into side of steering column.I pushed and waggled this. Another Yellow one points forward towards engine(push and waggle again). My problem appears solved!!
Have I waggled all the multiplugs or are some still to be found?
Vauxhall dealer (Cars older than 6 years will be charged) quoted me £78 +vat for diagnostic and £130 +vat (labour and plug cost ) to rectify!!
Bernard
August 2009