yes, 'headunit' = CD tuner.
ok, 'roadmaster' is not a high quality brand as far as i know. more an ebay item. and if it's designed in the US (may not be), it may not have the appropriate radio sensitivity as a UK spec model.
chances are there's little you can do, and that the aerial is intact and functioning fine. best option is to buy a new, high quality headunit. you could even consider a (secondhand) original unit from a 156 (produced by blaupunkt).
