The antenna-amplifier is situated eighter, in the right-rear upright (between rear-door & -window) or under the cower between headrests and window (in the middle between rear speakers) you might be able to spot it from below.
The reception of an window-antenna is not nearly as good as an ordenary antenna, but of course more practical and not in risk of vandalisme.
Further more the amplifier often breaks down. (bad quality i guess!)
I have had two 155's...the first, a '96 2,0 16V, the amp. was located in the upright, the second (present) a '94 2,5 V6 it is under the cower.