Does anyone know why my Astra 1.8sri Y-plate has a loss of power when you use anything electrical?

If you use any of the following my car lights and all inside display%26#039;s dim and there is a quick loss of power. i.e





turn the steering wheel


use electric windows


heated window button


heating - higher the blower more you can notice





ive had a new





oil pump


alternator


battery tested


ECU


earth tested





none of these have worked. Any idea%26#039;s?


please be precise as alot of money has been spent for nothing as main dealers just seem to guess and then take your money.......