Did he install the Blaupunkt with a "Phantomspeiseadapter?
Has the tester a "Phantomspeisung"?
If not, you MUST have a very bad receiving, cause the amplifier of the antenna doesn't work!
Why doesn't he install an other antenna (for instance with magnetic foot) temporarily for testing?
Install the blaupunkt or the tester with a "Phantomspeiseadapter" and you will see, the received signal will be good.
by the way.... of course your antenna (or the cable) can be damaged too.......
If your mechanic means that (but I don't believe it!), why doesn't he change the antenna?

Greetings
Torsten