Radio Installation

[Graham Sheppard]

My 1989 S4 is in good original condition and I try to keep it that way. The
Toronto SQR 46 radio works well,  but the cassette not so well. I have
decided to put in a Bremen SQR 46DAB. I intend to retain the original BQB
80 amp and the loudspeakers (which I have refoamed). I have the power
connection, part 91164240000. The problem is the connection to the amp
thruogh the original lead which has an 8 din connection, while the Bremen
pre amp out has RCA plugs. I purchased an RCA to 8 DIN lead from the US
(after shipping and VAT was very expensive). Unfortunately it was wired
incorrectly for a BQB 80. Another connector, appropriate for Porsche cars
is available from Bergvill in the US, but would end up extremely expensive
after shipping etc. I have decided to make a connector, however, because of
the RCA connections on the radio there is no ground between the radio and
amp. I have looked at the wiring diagram  and it appears that the ground
for both goes to the same point. I propose to run a ground wire from the
radio ground, on the power side, to the original 8 DIN cable. Has anyone
done this or arrived at an alternative solution while retaining the
original amp?
Regards
Graham