you definitely need the pc5-90 for a mk4 golf
previous post you linked a pc5-91

i have a pc5-90 installed at the moment and still getting utterly crap signal.
anyone with a mk4 who lives in the darlington or middlesbrough area fancy doing me a favor and quickly removing the head unit one night so i can plug mine in see if it works in your car

if not i know that its defanitly the head unit and can be sent back, if not then i defanitly know its something with the arial/arial circuit.
also had a bit of a look around on the internet, i dont know if its a small amplifier in the base of the arial but theres defanitly a circuit in there and theres a few things about them being replaced, and sorting out crap signal.