  The line of sight to the quasar B0218+357, one of the most studied lensed
systems, intercepts a z=0.68 spiral galaxy, which splits its image into two
main components A and B, separated by ca. 0.3'', and gives rise to molecular
absorption. Although the main absorption component has been shown to arise in
front of image A, it is not established whether some absorption from other
velocity components is also occuring in front of image B. To tackle this
question, we have observed the HCO+(2-1) absorption line during the
commissioning phase of the new very extended configuration of the Plateau de
Bure Interferometer, in order to trace the position of the absorption as a
function of frequency. Visibility fitting of the self-calibrated data allowed
us to achieve position accuracy between ~12 and 80 mas per velocity component.
Our results clearly demonstrate that all the different velocity components of
the HCO+(2-1) absorption arise in front of the south-west image A of the
quasar. We estimate a flux ratio fA/fB = 4.2 (-1.0;+1.8 at 106 GHz.
