Hi there ,i have just done a lcd replacement on my Audi TT2003 instament cluster , i reconnected the battery and the red battery symbol came on and the car only cranked once until fully flat battery !

try replacing the battery it likely got shot from the issues you had had. aside from that make sure the security is working right. when you put the key in does the blue symble come on and flash ? this is the issue with dash clusters in these cars. they often often after repairs don't work due to needing to be rekeyed to your car due to security.